Write the report until 5 pm.
Write the report by 5 pm.
Write the report until 5 pm means “Start writing the report now, continue writing it until 5 pm, then stop writing it regardless of whether it is finished or not.”
Write the report by 5 pm means “Make sure that at 5 pm the report is written and finished”.
As a preposition of time “by” means “on or before”.
E.g. We will send you the documents by Friday.
- Use “by” when you refer to a deadline.
“Until” means “up to a particular time”.
E.g. We have until Friday to send out the documents.
- Use “until” when you refer to the period of time before a deadline.
“By” is used to describe an action or event that happens on or before a particular moment.
E.g. We expect to know the judgment by the end of November.
“Until” is used to describe a state or situation in the period of time up to a particular moment.
E.g. The court proceedings are expected to last until November.
You may think of “until” as referring to the moment a state or situation changes.
E.g. Comments on the draft may be submitted until 30 April 2012.
Here’s a summary:
1. I can do it by 5 pm.
Now → do it and finish it → 5 pm (it will be finished)
2. I can do it until 5 pm.
Now → do it → 5 pm (I’ll stop even if I haven’t finished)
3. I can’t do it by 5 pm.
Now → do it / not do it → 5 pm (either way it won’t be finished)
4. I can’t do it until 5 pm.
Now → can’t do it → 5 pm (I’ll start)