Both until and by indicate “any
time before, but not later than a certain point in time.”
Until
tells us how long a situation continues.
If something happens until a particular time, you stop
doing it at that time.
For example:
They lived in a small house until September 2003.
(They stopped living there in September.)
I will be away until Wednesday.
(I will be back on Wednesday.)
We also use until in negative sentences.
For example:
Details will not be available until January.
(January is the earliest you can expect to receive the details.)
If something happens by a particular time, it happens at
or before that time. It is often used to indicate a deadline.
For example:
You have to finish by August 31.
(August 31 is the last day you can finish; you may finish before this
date.)
We also use by when asking questions.
For example:
Will the details be available by December?
(This asks if they will be ready no later than December.)
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