Both "until" and "by" indicate: any time before, but not later than.
Until tells us how long a situation continues until it finishes.
If something happens until a particular time, it means that at that time you stop doing it.
i.e:
They lived in Oxford Road 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.
i.e:
Details will not be available until January.
(January is the earliest date 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 (fecha límite).
i.e:
You have to finish by August 31.
(August 31 is the last day you can finish; you may finish before this date, but no after.)
-We also use "by" when asking questions.
i.e:
Will the details be available by December?
Got it?? Here you have an exercise to check if you understood. Good luck!
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-45609.php
