<p>Just to expand on Hugh’s answer a bit.</p>

<p>To understand what’s wrong with ‘我作天去商店.’ standing alone, we could translate it as ‘Yesterday I was going to the shop.’  Speaking English, if you said this and just stopped, the listener would think, well so what?</p>

<p>There are some verbs which are not used with 了 where a time phrase is enough to show past action.  For example, stative verbs:</p>

<blockquote>
  <p>我上個月不忙.   Last month I wasn’t busy.</p>
</blockquote>

<p>Also equative verbs like 是:</p>

<blockquote>
  <p>小的時候他是我的朋友.  He was my friend when we were kids.</p>
</blockquote>

<p>And also with some regular verbs:</p>

<blockquote>
  <p>去年他住在北京.   Last year he lived/was living in Beijing.</p>
  
  <p>昨天我想家.   Yesterday I was missing home.</p>
</blockquote>

<p>But in all these cases, the action described is inherently incomplete.  So Hugh is correct, it’s best to go verb by verb and get to know how frequently and in what situations each will be used for completed actions.</p>

<p>If you want another take on aspect, try studying Russian with its perfective/imperfective verb system.  For a native speaker of English, it’s really hard to get your mind around.</p>

CC BY-SA 3.0
2014-02-13T14:53:06.360Z
Thanks! It makes sense. When you say that in these sentences the action is "inherently incomplete", is it that in those situations the verb is not something you do and complete quickly? Like the verb 是 is not an action that gets completed. Even 住 is not a short term action that is done and completed.
Jon Hargett
CC BY-SA 3.0
2014-02-13T16:24:28.967Z
Yes, that’s what I meant.  Other verbs, like 到, usually show completed action – when you arrive, you arrive – so they’ll typically have 了.
neubau
CC BY-SA 3.0
2014-02-19T04:32:23.833Z
So is e.g. 住了 (aspectual 了) not possible at all, or just not necessary when there's a time phrase?
dainichi
CC BY-SA 3.0
2014-02-22T04:08:32.090Z
@dainichi:  Yes, I think 住了is OK in a construction like this: 我在北京住了一年多.  ‘I lived in Beijing for more than a year.’  I am not a native speaker, however.
neubau
CC BY-SA 3.0
2014-02-22T04:09:09.403Z
For more examples of verb 了 followed by a complement of duration, see Yip and Rimmington’s ‘Comprehensive Grammar’, sect. 7.2.
neubau
CC BY-SA 3.0
2014-02-13T14:30:00.687Z
2014-02-13T14:30:00.687Z
neubau
<p>“了” is a special character usually meaning &quot;finished&quot; or &quot;something happen in the future&quot;, we can summarize these points:</p>
<ol>
<li>For a verb that can be persistent, “了” means “begin to do something immediately on spoken” （usually SVO+了）：</li>
</ol>
<p>妈妈，我<strong>做功课了</strong>；做完功课后我出去散步。(Mon, I'll <strong>start homeworking</strong>, then I'll walk outside).</p>
<p>However, <strong>if SV了+O（Subject+Verb+了+Object），mostly this means something happened</strong>：</p>
<p>Compare with: 妈妈，我<strong>做了功课</strong>。</p>
<ol start="2">
<li>For a verb that can NOT be persistent, &quot;了&quot; means <strong>a finished state</strong>：</li>
</ol>
<p>他<strong>坐下了</strong>。 - He <strong>has sit down</strong> (implying that he's currently sitting).</p>
<p>他<strong>骨折了</strong>。 - He <strong>has had a fracture</strong> (implying that the fracture is still there).</p>
<p>树叶<strong>掉落了</strong>。 - Leaves <strong>have fallen</strong> (implying that they are still on the ground).</p>
<p>3）For a verb that can be persistent but intransitive，&quot;了&quot; means something happened or immediately happen (just depending on contexts):</p>
<p>他唱了。(He's sung/He's going to sing).</p>

CC BY-SA 3.0
2014-02-12T14:01:31.593Z
Nice information, but I am only concerned with the aspect marker. See edit to clarify my question.
Jon Hargett
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2014-02-12T20:58:29.017Z
I don't agree with some of this or maybe I am just not understanding the English. I will try to edit this a bit later.
deutschZuid
CC BY-SA 3.0
2014-02-13T03:45:44.427Z
"Subject+Verb+了+Object，mostly this means something happened" You could use that structure to talk about events in the future though, e.g. "我吃了饭要出去玩儿。"
MHG
CC BY-SA 4.0
2014-02-12T06:42:12.217Z
2021-08-22T12:03:01.527Z
2021-08-22T12:03:01.527Z
xqMogvKW
<p>1.When "了" put the end of a sentence, that means something changed/be changing/be going to change. It can be used in the past, present and future. But usually there is "就" in the future tense. "就" is similar to "will". for example:</p>

<p>"昨天，我去学校了。" refers to "I went to school yesterday."</p>

<p>"现在，我去学校了。" refers to "I am going to school now."</p>

<p>"明天，我就去学校了。" refers to "I will go to school tomorrow."</p>

<p>2.When "了" follows a Verb, that means something had done. And it should only be used in the past tense.for instance:</p>

<p>"昨天，我去了学校。" refers to "I went to school yesterday."</p>

<p>3.If a sentence without a "了", that means doing something/be going to do something. And it could be used in the present tense and future tense. for example:</p>

<p>"你现在去哪里？-- 我现在去学校。" means "Where are you going now? -- I'm going to school now."</p>

<p>"你明天去哪里？-- 我明天去学校。" means "Where will you go tomorrow? -- I will go to school tomorrow."</p>

CC BY-SA 3.0
2014-02-12T14:04:20.010Z
Thanks. I'm only concerned with #2 here. My question is can I leave off 了 if the time word is there. See edit for my clarification.
Jon Hargett
CC BY-SA 3.0
2014-02-12T14:31:57.817Z
@JonHargett It is not exactly that leaving off "了". It is better to add what you did in the grocery shop. Just like "我昨天去商店买面包"
zz22
CC BY-SA 3.0
2014-02-12T15:46:32.450Z
What if I am private and I don't want to tell you what I did at the store. :)
Jon Hargett
CC BY-SA 3.0
2014-02-13T00:41:36.240Z
@JonHargett ooops. I forgot a "correct" in the last reply. I wanted to say "It is not correct exactly that leaving off".
zz22
CC BY-SA 3.0
2014-02-12T12:24:15.977Z
2014-02-12T12:24:15.977Z
zz22
<blockquote>
  <p>Is it grammatically incorrect to say ”我昨天去商店“？</p>
</blockquote>

<p>It is grammatically correct. Since the sentence already indicate past tense you don't have to use it.</p>

<p>In basic Chinese learning material it is common to avoid using le 了 to keep it simple. See the examples at <a href="http://www.dictall.com/indu57/08/5708187C16D.htm" rel="nofollow">http://www.dictall.com/indu57/08/5708187C16D.htm</a>
(maybe you can find some in one of your learning books to verify)</p>

<blockquote>
  <p>Since "yesterday" is included in the sentence, does 了 really add anything?</p>
</blockquote>

<p>Depends on the context. Probably not in this example, but 了 can also be used to make a sentence sound more serious. For example when saying 谢谢 you can add 了 - "谢谢了" to indicate that you're sincerely grateful.</p>

<p>Note that I got this information from a long discussion with a Chinese friend - I've got no credible sources to link to. Hopefully someone else can contribute with a credible source. </p>

CC BY-SA 3.0
2014-02-12T20:11:29.640Z
Thanks for the link! I have been looking through all of my books and I could not find an example where a past time word was present and 了 was NOT used, although I was sure I read that somewhere in the past. It helped to see it somewhere else. I am inclined to mark this as the answer unless someone has a more credible reference as you suggest.
Jon Hargett
CC BY-SA 3.0
2014-02-12T19:41:15.457Z
2014-02-12T19:41:15.457Z
Niklas Berglund
<h3>Edit for your new question.</h3>

<ul>
<li><p>我昨天去商店。Normally when you said it without 了， this sentence does not stop, you'd like to continuous and introduce what you did in the shop. </p></li>
<li><p>我昨天去了商店。 That's end. I got the information, you have been to shop. You can continuous your topic or not, what ever. </p></li>
</ul>

CC BY-SA 3.0
2014-02-12T13:04:19.507Z
He said in the question that the time is already included in the sentence, so 了 does not contribute anything to the meaning of his example sentence.
Niklas Berglund
CC BY-SA 3.0
2014-02-12T14:40:58.307Z
Thank you @user3945 and Niklas you are answering my question directly. Unfortunately, you have two opposing answers!
Jon Hargett
CC BY-SA 3.0
2014-02-12T18:33:01.227Z
@JonHargett it was not an answer to the question, I was trying to explain that in your example sentence 了 don't contribute to the sentence, thus you're asking whether it's necessary to include.
Niklas Berglund
CC BY-SA 3.0
2014-02-13T02:08:43.147Z
Thanks, as this does address my question. But it does seem to contradict @NiklasBerglund 's answer. Any references?
Jon Hargett
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2014-02-14T12:03:20.890Z
@Jon, I saw you have accepted Hugh's answer.  Yes, that's right, he got my points, and express in English in better way.
Bill
CC BY-SA 3.0
2014-02-12T04:04:04.140Z
2014-02-15T00:48:38.130Z
2014-02-15T00:48:38.130Z
Bill
<p>了 is a special word.</p>

<p>There are 3 question have to ask first.</p>

<pre><code>A. 了 is a verb or not?
B. If not, is there other verb in the sentence?
C. If so, 了 is after the verb? or in the end of the sentence?
</code></pre>

<p>Following your example,</p>

<pre><code>了 is not a verb.
了 is after a verb,
</code></pre>

<p>There are 3 tense I know.</p>

<ol>
<li><p>past tense</p>

<pre><code>我昨天去商店。
</code></pre>

<p>This is not a complete sentence!</p>

<p>People usually say that following something else.</p>

<p>For example: 我昨天去商店，買了很多東西。</p>

<p>You can simply say that, but it's weird.</p>

<p>(You said that, then? What'd happened?)</p>

<p>The special situation is when you are in a conversation:</p>

<pre><code>example 1:
甲：我昨天去商店。(Yesterday, I went to the store.)
乙：去幹嘛？(So?)
甲：你猜！(You guess?)
......(a long conversation.)
</code></pre>

<p>.</p>

<pre><code>example 2:
甲：你昨天幹嘛？(What did you do yesterday?)
乙：我昨天去商店。
</code></pre>

<p>.</p>

<pre><code>我昨天去了商店。
</code></pre>

<p>Same as non-了 sentence, but emphasize it like "already did".</p></li>
<li><p>present tense</p>

<pre><code>我現在去商店。
</code></pre>

<p>Not a complete sentence! Same reason as past tense.</p>

<p>But you can't use 了 in this sentence if you wanna say</p>

<p>"I'm going to the store now."</p>

<p>You may use 就 to emphasize it like "我現在就去商店。"</p>

<pre><code>我現在去了商店。
</code></pre>

<p>I'm on my way to the store.</p>

<p>It's present and use 了 to tell someone in the past I was doing this too,</p>

<p>It's Present Continuous! Same meaning as "我正在去商店的路上".</p></li>
<li><p>future tense</p>

<pre><code>我明天去商店。
</code></pre>

<p>This is not a complete sentence! Same as present-tense.</p>

<pre><code>我明天去了商店。
</code></pre>

<p>It's weird, because 了 is past and 明天 is future, but may happen in a story of a book.</p></li>
</ol>

CC BY-SA 3.0
2014-02-13T02:49:03.533Z
2014-02-13T02:54:57.807Z
2014-02-13T02:54:57.807Z
c-s.c
<p>I think Chinese textbooks should start their 了 sections with this:</p>

<ol>
<li>了 is not about time.</li>
<li>了 is not about tense.</li>
<li>Goto 1.</li>
</ol>

<p>You are only concerned with 了 as an the aspect marker, aka completed action 了, or perfect aspect 了, so:</p>

<p>"昨天去商店" and "昨天去了商店" are both valid <strong>verb phrases</strong>. The second one explicitly states that the action was completed, whereas the first one does not.</p>

<p>As other posters have mentioned, "我昨天去商店" isn't wrong, but it is incomplete as a sentence because it leaves that verb phrase hanging. It feels like "Yesterday I went to the shop and..." in English. It needs to carry on with more information, e.g. "我昨天去商店的时候发现鸡蛋都卖完了".</p>

<p>"我昨天去了商店" is a complete sentence.</p>

<p>It might be easiest to look at examples to get a better sense of this:</p>

<p>我们昨天卖了十个。 -> "We sold ten yesterday."</p>

<p>到了红绿灯，往左拐。 -> "When you get to the traffic lights, turn left."</p>

<p>你买了东西以后可以去银行取点现金吗？ -> "After you've bought the things could you go to the bank and get some cash out?" (notice this in the future, the action hasn't happened yet, but still uses 了)</p>

<p>My point is that 了 is not about tense or time. It's about whether an action is complete <strong>in the time frame we're talking about</strong>.</p>

<p>I think that this is the sort of issue that can't be resolved by reading textbooks or explanations online. It's got to be done through long-term exposure and practice, in my view.</p>

CC BY-SA 3.0
2014-02-13T04:47:09.137Z
Thanks everyone for your thoughts. This answer made the most sense to me so I am marking it as the answer. Just one last thought. I get that 了 is about a completed action. I just figured that if it happened in the past and I say "go", as in I'm not currently there, then it is obvious that I completed the trip. Maybe it is obvious to me but not the Chinese listener.
Jon Hargett
CC BY-SA 3.0
2014-02-13T03:36:35.013Z
2014-02-13T04:55:23.927Z
2014-02-13T04:55:23.927Z
MHG
<p>In sentences that already have a point in time word that indicates that the event happened in the past, is 了 optional? Or could there be some additional meaning with it present? For instance:</p>

<blockquote>
  <p>我昨天去商店。</p>
</blockquote>

<p>or</p>

<blockquote>
  <p>我昨天去了商店。</p>
</blockquote>

<p>Since "yesterday" is included in the sentence, does 了 really add anything?</p>

<p>Edit:</p>

<p>I am not concerned with the end-of-sentence 了 in this question, only the aspect marker. Follow up question: Is it grammatically incorrect to say ”我昨天去商店“？ That is, it obviously happened in the past but i don't use 了。</p>

CC BY-SA 3.0
2014-02-12T07:11:54.723Z
I'm pretty sure that `了` is not about [past tense](https://en.wikipedia.org/wiki/Past_tense) but about [perfective aspect](https://en.wikipedia.org/wiki/Perfective_aspect). Those are two specialized grammatical terms. Often languages only have either tense or aspect or not both and many uses of the past tense, but not all, will map to uses of the perfective aspect in other languages.
hippietrail
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2014-02-12T13:05:55.123Z
Is it just me, or is it a trend on Chinese SE to write unnecessarily long answers?
Niklas Berglund
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2014-02-12T13:57:38.310Z
@NiklasBerglund: Maybe you can also submit a concise answer, with only the necessary information?
hippietrail
CC BY-SA 3.0
2014-02-12T18:30:00.797Z
@hippietrail That was my understanding as well. the time word indicates the tense of the sentence, so what does the 了 do? Generally indicate that the action is complete, but it should be obvious that it is complete if you went yesterday and you are no longer there when you say it!
Jon Hargett
CC BY-SA 3.0
2014-02-12T18:38:22.203Z
@hippietrail It's a good idea, and I'd love to. I don't know the answer to this question though.
Niklas Berglund
<grammar><past-tense><particles>
Is 了 optional when a time word is present?
# Is 了 optional when a time word is present?

(Asked by: username_0 on 2014-02-12)

In sentences that already have a point in time word that indicates that the event happened in the past, is 了 optional? Or could there be some additional meaning with it present? For instance:

> 我昨天去商店。

or

> 我昨天去了商店。

Since "yesterday" is included in the sentence, does 了 really add anything?

Edit:

I am not concerned with the end-of-sentence 了 in this question, only the aspect marker. Follow up question: Is it grammatically incorrect to say ”我昨天去商店“？ That is, it obviously happened in the past but i don't use 了。

**Tags:** grammar, past-tense, particles

> **Comments:**
> **username_1**: I'm pretty sure that `了` is not about [past tense](https://en.wikipedia.org/wiki/Past_tense) but about [perfective aspect](https://en.wikipedia.org/wiki/Perfective_aspect). Those are two specialized grammatical terms. Often languages only have either tense or aspect or not both and many uses of the past tense, but not all, will map to uses of the perfective aspect in other languages.  
> **username_2**: Is it just me, or is it a trend on Chinese SE to write unnecessarily long answers?  
> **username_1**: @NiklasBerglund: Maybe you can also submit a concise answer, with only the necessary information?  
> **username_0**: @username_1 That was my understanding as well. the time word indicates the tense of the sentence, so what does the 了 do? Generally indicate that the action is complete, but it should be obvious that it is complete if you went yesterday and you are no longer there when you say it!  
> **username_2**: @username_1 It's a good idea, and I'd love to. I don't know the answer to this question though.  

---

## 7 answers

### Answer by username_7 on 2014-02-13. Score: 13 | Accepted answer

I think Chinese textbooks should start their 了 sections with this:

1. 了 is not about time.
2. 了 is not about tense.
3. Goto 1.

You are only concerned with 了 as an the aspect marker, aka completed action 了, or perfect aspect 了, so:

"昨天去商店" and "昨天去了商店" are both valid **verb phrases**. The second one explicitly states that the action was completed, whereas the first one does not.

As other posters have mentioned, "我昨天去商店" isn't wrong, but it is incomplete as a sentence because it leaves that verb phrase hanging. It feels like "Yesterday I went to the shop and..." in English. It needs to carry on with more information, e.g. "我昨天去商店的时候发现鸡蛋都卖完了".

"我昨天去了商店" is a complete sentence.

It might be easiest to look at examples to get a better sense of this:

我们昨天卖了十个。 -> "We sold ten yesterday."

到了红绿灯，往左拐。 -> "When you get to the traffic lights, turn left."

你买了东西以后可以去银行取点现金吗？ -> "After you've bought the things could you go to the bank and get some cash out?" (notice this in the future, the action hasn't happened yet, but still uses 了)

My point is that 了 is not about tense or time. It's about whether an action is complete **in the time frame we're talking about**.

I think that this is the sort of issue that can't be resolved by reading textbooks or explanations online. It's got to be done through long-term exposure and practice, in my view.

> **Comments:**
> **username_0**: Thanks everyone for your thoughts. This answer made the most sense to me so I am marking it as the answer. Just one last thought. I get that 了 is about a completed action. I just figured that if it happened in the past and I say "go", as in I'm not currently there, then it is obvious that I completed the trip. Maybe it is obvious to me but not the Chinese listener.  

---

### Answer by username_3 on 2014-02-13. Score: 9

Just to expand on Hugh’s answer a bit.

To understand what’s wrong with ‘我作天去商店.’ standing alone, we could translate it as ‘Yesterday I was going to the shop.’ Speaking English, if you said this and just stopped, the listener would think, well so what?

There are some verbs which are not used with 了 where a time phrase is enough to show past action. For example, stative verbs:

> 我上個月不忙. Last month I wasn’t busy.

Also equative verbs like 是:

> 小的時候他是我的朋友. He was my friend when we were kids.

And also with some regular verbs:

> 去年他住在北京. Last year he lived/was living in Beijing.
>
> 昨天我想家. Yesterday I was missing home.

But in all these cases, the action described is inherently incomplete. So Hugh is correct, it’s best to go verb by verb and get to know how frequently and in what situations each will be used for completed actions.

If you want another take on aspect, try studying Russian with its perfective/imperfective verb system. For a native speaker of English, it’s really hard to get your mind around.

> **Comments:**
> **username_0**: Thanks! It makes sense. When you say that in these sentences the action is "inherently incomplete", is it that in those situations the verb is not something you do and complete quickly? Like the verb 是 is not an action that gets completed. Even 住 is not a short term action that is done and completed.  
> **username_3**: Yes, that’s what I meant.  Other verbs, like 到, usually show completed action – when you arrive, you arrive – so they’ll typically have 了.  
> **username_4**: So is e.g. 住了 (aspectual 了) not possible at all, or just not necessary when there's a time phrase?  
> **username_3**: @username_4:  Yes, I think 住了is OK in a construction like this: 我在北京住了一年多.  ‘I lived in Beijing for more than a year.’  I am not a native speaker, however.  
> **username_3**: For more examples of verb 了 followed by a complement of duration, see Yip and Rimmington’s ‘Comprehensive Grammar’, sect. 7.2.  

---

### Answer by username_5 on 2014-02-12. Score: 3

“了” is a special character usually meaning "finished" or "something happen in the future", we can summarize these points:

1. For a verb that can be persistent, “了” means “begin to do something immediately on spoken” （usually SVO+了）：

妈妈，我**做功课了**；做完功课后我出去散步。(Mon, I'll **start homeworking**, then I'll walk outside).

However, **if SV了+O（Subject+Verb+了+Object），mostly this means something happened**：

Compare with: 妈妈，我**做了功课**。

2. For a verb that can NOT be persistent, "了" means **a finished state**：

他**坐下了**。 - He **has sit down** (implying that he's currently sitting).

他**骨折了**。 - He **has had a fracture** (implying that the fracture is still there).

树叶**掉落了**。 - Leaves **have fallen** (implying that they are still on the ground).

3）For a verb that can be persistent but intransitive，"了" means something happened or immediately happen (just depending on contexts):

他唱了。(He's sung/He's going to sing).

> **Comments:**
> **username_0**: Nice information, but I am only concerned with the aspect marker. See edit to clarify my question.  
> **username_6**: I don't agree with some of this or maybe I am just not understanding the English. I will try to edit this a bit later.  
> **username_7**: "Subject+Verb+了+Object，mostly this means something happened" You could use that structure to talk about events in the future though, e.g. "我吃了饭要出去玩儿。"  

---

### Answer by username_8 on 2014-02-12. Score: 1

1.When "了" put the end of a sentence, that means something changed/be changing/be going to change. It can be used in the past, present and future. But usually there is "就" in the future tense. "就" is similar to "will". for example:

"昨天，我去学校了。" refers to "I went to school yesterday."

"现在，我去学校了。" refers to "I am going to school now."

"明天，我就去学校了。" refers to "I will go to school tomorrow."

2.When "了" follows a Verb, that means something had done. And it should only be used in the past tense.for instance:

"昨天，我去了学校。" refers to "I went to school yesterday."

3.If a sentence without a "了", that means doing something/be going to do something. And it could be used in the present tense and future tense. for example:

"你现在去哪里？-- 我现在去学校。" means "Where are you going now? -- I'm going to school now."

"你明天去哪里？-- 我明天去学校。" means "Where will you go tomorrow? -- I will go to school tomorrow."

> **Comments:**
> **username_0**: Thanks. I'm only concerned with #2 here. My question is can I leave off 了 if the time word is there. See edit for my clarification.  
> **username_8**: @JonHargett It is not exactly that leaving off "了". It is better to add what you did in the grocery shop. Just like "我昨天去商店买面包"  
> **username_0**: What if I am private and I don't want to tell you what I did at the store. :)  
> **username_8**: @JonHargett ooops. I forgot a "correct" in the last reply. I wanted to say "It is not correct exactly that leaving off".  

---

### Answer by username_2 on 2014-02-12. Score: 1

> Is it grammatically incorrect to say ”我昨天去商店“？

It is grammatically correct. Since the sentence already indicate past tense you don't have to use it.

In basic Chinese learning material it is common to avoid using le 了 to keep it simple. See the examples at <http://www.dictall.com/indu57/08/5708187C16D.htm>
(maybe you can find some in one of your learning books to verify)

> Since "yesterday" is included in the sentence, does 了 really add anything?

Depends on the context. Probably not in this example, but 了 can also be used to make a sentence sound more serious. For example when saying 谢谢 you can add 了 - "谢谢了" to indicate that you're sincerely grateful.

Note that I got this information from a long discussion with a Chinese friend - I've got no credible sources to link to. Hopefully someone else can contribute with a credible source.

> **Comments:**
> **username_0**: Thanks for the link! I have been looking through all of my books and I could not find an example where a past time word was present and 了 was NOT used, although I was sure I read that somewhere in the past. It helped to see it somewhere else. I am inclined to mark this as the answer unless someone has a more credible reference as you suggest.  

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### Answer by username_9 on 2014-02-12. Score: 1

### Edit for your new question.

* 我昨天去商店。Normally when you said it without 了， this sentence does not stop, you'd like to continuous and introduce what you did in the shop.
* 我昨天去了商店。 That's end. I got the information, you have been to shop. You can continuous your topic or not, what ever.

> **Comments:**
> **username_2**: He said in the question that the time is already included in the sentence, so 了 does not contribute anything to the meaning of his example sentence.  
> **username_0**: Thank you @user3945 and Niklas you are answering my question directly. Unfortunately, you have two opposing answers!  
> **username_2**: @JonHargett it was not an answer to the question, I was trying to explain that in your example sentence 了 don't contribute to the sentence, thus you're asking whether it's necessary to include.  
> **username_0**: Thanks, as this does address my question. But it does seem to contradict @NiklasBerglund 's answer. Any references?  
> **username_9**: @Jon, I saw you have accepted Hugh's answer.  Yes, that's right, he got my points, and express in English in better way.  

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### Answer by username_10 on 2014-02-13. Score: 0

了 is a special word.
There are 3 question have to ask first.
A. 了 is a verb or not?
B. If not, is there other verb in the sentence?
C. If so, 了 is after the verb? or in the end of the sentence?

Following your example,
了 is not a verb.
了 is after a verb,

There are 3 tense I know.

past tense
我昨天去商店。

This is not a complete sentence!
People usually say that following something else.
For example: 我昨天去商店，買了很多東西。
You can simply say that, but it's weird.
(You said that, then? What'd happened?)
The special situation is when you are in a conversation:
example 1:
甲：我昨天去商店。(Yesterday, I went to the store.)
乙：去幹嘛？(So?)
甲：你猜！(You guess?)
......(a long conversation.)

.
example 2:
甲：你昨天幹嘛？(What did you do yesterday?)
乙：我昨天去商店。

.
我昨天去了商店。

Same as non-了 sentence, but emphasize it like "already did".
present tense
我現在去商店。

Not a complete sentence! Same reason as past tense.
But you can't use 了 in this sentence if you wanna say
"I'm going to the store now."
You may use 就 to emphasize it like "我現在就去商店。"
我現在去了商店。

I'm on my way to the store.
It's present and use 了 to tell someone in the past I was doing this too,
It's Present Continuous! Same meaning as "我正在去商店的路上".
future tense
我明天去商店。

This is not a complete sentence! Same as present-tense.
我明天去了商店。

It's weird, because 了 is past and 明天 is future, but may happen in a story of a book.


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