By vs Until
Level B1 · Prepositions
by and until both refer to time, but they describe different kinds of deadlines. by means 'at or before a specific point' — the action happens once, at some moment up to that time: Submit the form by Friday (any time before or on Friday, but done by then). until means 'up to and continuing to that point' — an ongoing situation or state that stops exactly there: I'll be at work until Friday (working continuously, ending on Friday).
How to choose between by and until:
- by + a deadline, with a one-time action or completed event: Finish the report by 5pm (complete it at some point no later than 5); The package should arrive by Monday.
- until (or till, informal) + a point in time, with a continuous or repeated situation that stops at that point: She stayed at the party until midnight (continuously there, then left); We won't know the results until Friday (we remain without knowing, right up to Friday).
- A useful test: if you can insert 'no later than' and the meaning still works, use by; if you can insert 'up to the moment when' and the meaning still works, use until: Pay by the 1st (no later than the 1st) vs Keep the oven on until it beeps (up to the moment it beeps).
- by has a second, unrelated meaning — 'by a margin of', giving the size of a difference or change, typically with a comparative or a verb of increase/decrease: prices rose by 10%; she won the race by two seconds; this room is bigger by a metre. Tell the two apart by what the by answers: 'by when?' is the deadline sense, 'by how much?' is this one.
The most common mistake is using until with a one-time completed action — Finish the report until Friday sounds wrong; a single completion needs by: Finish the report by Friday. A second mistake is using by with a continuous situation that should last right up to a point — I'll wait by 5 o'clock is unnatural; a continuous wait needs until: I'll wait until 5 o'clock.
Ask whether the sentence describes a single event that must happen no later than a certain time (use by) or a state/situation that continues right up to that time and then stops (use until). If you can replace the word with 'no later than', it's by; if you can replace it with 'up to the moment when', it's until.