Q: Should I use 'by' as a preposition of time in past/future perfect tenses only? In which tenses can I use 'by' as a preposition of time? My teacher told me you are allowed to use this preposition in past perfect or future perfect tenses only, but I ran into a lot of sentences on the web where people use it in past or future simple/continuous tenses. Consider the following examples:

I will do my homework by tomorrow.
  I will have done my homework by tomorrow.
They will be working by then.
      They will have been working by then.
The domestic water use did not change by 1960.
      The domestic water use had not changed by 1960.

Please let me know which of them are correct.

A: In that sense, you can use 'by' with any tense. Consider its meaning as given by the Longman dictionary: 

Before or not later than a particular time.

Examples: 


*

*The document needs to be ready by next Friday.

*By the end of the day we had sold over 2000 tickets.

*By the time we got home we were tired and hungry.

*I'll be home by 9:30.

*Please try to have this done by Friday.


All the examples are borrowed from the Longman dict. so you can trust them.
