By (+ a time) = not later than:
- I sent the letter to them today, so they should receive it by Monday.
(= on or before Monday, not later than Monday)
- We’d better hurry. We have to be home by 5 o’clock.
(= at or before 5 o’clock, not later than 5 o’clock)
- Where’s Sarah? She should be here by now.
(= now or before now – so she should have already arrived)
We use until (or till) to say how long a situation continues:
- ‘Shall we go now?’ ‘No, let’s wait until (or till) it stops raining.
- I couldn’t get up this morning.
I stayed in bed until half past ten./ I didn’t get up until half past ten.
Compare until and by:
Something continues until a time in the future:
- David will be away until Monday, (so he’ll be back on Monday)
- I’ll be working until 11.30. (so I’ll stop working at 11.30)
Something happens by a time in the future:
- David will be back by Monday.
(so he’ll be back not later than Monday)
- I’ll have finished my work by 11.30.
(= I’ll finish my work not later than 11.30.)
You can say ‘by the time something happens’.
- It’s too late to go to the bank now. By the time we get there, it will be closed.
(= the bank will close between now and the time we get there)
- (from a postcard) Our holiday ends tomorrow. So by the time you receive this postcard. I’ll be back home.
(= I will arrive home between tomorrow and the time you receive this postcard)
- Hurry up! By the time we get to the cinema, the film will already have started.
You can say ‘by the time something happened’ (for the past):
- Karen’s car broke down on the way to the party last night. By the time she arrived, most of the other guests had left.
(= it took her a long time to get to the party and most of the guests left during this timei
- I had a lot of work to do yesterday evening. I was very tired by the time I finished.
(= it took me a long time to do the work, and I became more and more tired during this time)
- We went to the cinema last night. It took us a long time to find somewhere to park the car. By the time we got to the cinema, the film had already started.
Also by then or by that time:
- Karen finally arrived at the party at midnight, but by then (or by that time), most of the guests had left.