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1- We danced until dawn.

2- She can't leave until Friday.

Is it possible to use "by" in 2? If no, why not? Which do you think should be used in 3, by or until?

3- He won't be back in the office ---- 4 o'clock.

What is the difference between 2 and 3?
Comments  
In #2, using "until," she will be ready to leave by Friday, but no sooner.

In #2, using "by," she won't be ready to leave on Friday - presumably when someone wanted to go. In fact, we don't know when she will be ready to go, but Friday is too soon. When you use "by," someone outside of the speaker has inquired about the availability of that time/date/day and you are responding to it.

In #3, assume someone is having a meeting at 4 and wants him to be there. If you say "he won't be back in the office by 4," you are saying he will not able to attend - and you are not giving any other information about when he WILL be back. On the other hand, someone wants a meeting with him and asks whether he's available that day, and you say "he won't be back in the office until 4," that means the person can schedule the meeting for 4 and know he'll be back for it.

If you use the "by" construction, it would be helpful to say when the person is going to be read/available/whater.

"She can't leave by Friday, but if you schedule the depature for Sunday, she can make it then."

Or "He won't be back in the office by 4, but if you can wait until 5, he'll be back then."
Thank you, Grammar Geek.
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some might find this useful:

http://www.longman.com/ae/azar/grammar_ex/message_board/archive/
select/click on "until"
click on "By vs. until"
Marius Hancusome might find this useful:

http://www.longman.com/ae/azar/grammar_ex/message_board/archive/
select/click on "until"
click on "By vs. until"
Thank you, Marius.Emotion: smile
So, is it correct to say "He will be back in the office until Monday"? or "He will not be back in the office until Monday"?
Students: We have free audio pronunciation exercises.
Anonymous
So, is it correct to say "He will be back in out of the office until Monday"? or "He will not be back in the office until Monday"? Okay

thanks . It is very helpful.