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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.

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    Will it already have been started before the time period? then use by. – SrJoven Aug 26 '14 at 14:29
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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.

  • Thanks a lot. Thus, with respect to @SrJoven's answer and what you explained above, the 'by' preposition of time can be used with any tenses. I wonder why my teacher warned me about that! – Mimi Aug 26 '14 at 16:32
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    @Mimi I'm not sure, but be careful. There are instances in which one cannot use 'by'. In fact I am not sure if your example of past tense use, 'Domestic water use did not change by 1960'. What exactly are you trying to say with that sentence? On its own it does not sound correct, but if you say '...did not change by 1960, as had been forecast', that sounds alright. Also some present tense applications do not work with 'by'. 'I am going to the shop by 3.00pm' does not sound right. One would normally say 'I am going to the shop 'before 3.00pm'. – WS2 Aug 26 '14 at 16:55
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    ...True. And 'They will have been working by then.' needs careful contextualising, such as '6 o'clock? They will have been working for 9 hours by then.' – Edwin Ashworth Aug 26 '14 at 18:45
  • @Mimi> You're most welcome, and I don't know why your teacher has said so, but perhaps he/she had something in mind when he/she was telling you this. – mok Aug 26 '14 at 19:22
  • @WS2 thank you for your informative comment. I wrote the sentence to describe this graph. The complete sentence is: The domestic water use did not change by 1960, then it rose from 100 km3 to almost 300 km3 in 2000. My teacher corrected my sentence as: The domestic water use had not changed by 1960 ... – Mimi Aug 26 '14 at 20:42

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