Use by for a deadline: the action happens at some point before then. Send it by Friday. Use until for a situation that continues all the time up to then: I'll be away until Friday.
Same situation, two choices. Pick an example.
Please send the report by Friday.
No later than: it happens once, at any time before the line.
I'll be working on the report until Friday.
Without stopping up to that moment, and then it ends.
| Rule | By | Until |
|---|---|---|
| Meaning | No later than: a deadline | The whole time up to a point |
| The action | Happens once, before the line | Continues, then stops |
| Typical verbs | send, finish, arrive, be ready | wait, stay, work, be open |
| With not | Not by Friday: it'll be late | Not until Friday: it only starts then |
The essay must be submitted _____ Monday.
Paste an email or an essay. Clarigram marks the mistakes you actually make, shows the rule behind each one as a picture, and builds your practice from them.
Till is a shorter, slightly informal form of until. Both are correct. Avoid the spelling 'til in formal writing.
It works like by, followed by a clause: By the time you arrive, I'll have finished. Use the present for the future after it, not will.