English sentences can be classified four different ways according to their structure: simple, compound, complex and compound-complex. The classification depends on the number of independent and dependent clauses a sentence contains. An independent clause is a complete sentence. It can stand on its own. A dependent clause is not a complete sentence. It must be joined with an independent clause to form a complete sentence. By learning these types and how they can (and cannot) be combined, you’ll be able to greatly increase the complexity and variety of your sentences.
A simple sentence contains one independent clause and no dependent clauses:
I need to learn English
I take English classes at the English Island in Atlanta.
A compound sentence contains two independent clauses but no dependent clauses:
I went to the grocery store, and I bought milk.
You can join the independent clauses with either a semi-colon (;) or a comma (,) followed by a conjunction (and, or, but). You cannot join the clauses with a comma by itself. This grammar error (which is relatively common among native English speakers) is called a comma splice.
I went to Publix to buy milk; the store was very busy.
I need to buy milk, but the grocery store is closed.
I need to buy milk the grocery store is very busy (Run-on sentence with no punctuation between clauses.)
I need to buy milk, the grocery is very busy (Comma splice)
A complex sentence contains an independent clauses and at least one dependent clause:
After taking classes at the English Island, I feel more confident speaking during meetings.
I feel more confident speaking during meetings after taking classes at the English Island.
When the dependent clause comes at the beginning of a sentence, it is usually followed by a comma. The comma may occasionally be omitted if the dependent clause is essential to the meaning of the sentence.
A complex-compound sentence combines the elements of a complex and a compound sentence. It has two independent clauses and at least one independent clause:
After taking classes at the English Island, I feel much more confident speaking during meetings; furthermore, I received a promotion for the excellent presentation that I delivered.
I received a promotion for the excellent presentation that I delivered after taking classes at the English Island, and I feel more confident speaking up during meetings in general.