Until and By — English Grammar Exercises
at, on, in, for, since, during, until, by — every time preposition in one place
Until and By — Continuity vs Deadline
Until and by are both used with future time expressions but describe fundamentally different relationships between an action and a point in time. Confusing them produces unnatural or misleading sentences. Learner corpus data indicates that until/by confusion is most acute at B1 level, particularly in formal writing contexts where deadline language is common.
Until — Continuous Action to a Cutoff
Until (or till) describes a state or action that continues without interruption and then stops at a specific point:
I'll wait for you until you arrive.
She worked until midnight and then went to bed.
By — Deadline for Completion
By marks a deadline — the point before or at which something must be done. The action itself may happen at any time before that point:
The project must be completed by the end of the month.
Please send me the documents by tomorrow.
The Paraphrase Test
If you can replace the preposition with 'no later than', use by. If the sentence describes a continuous state that stops at a point, use until:
'The shop stays open and stops at 10 pm' → open until 10 pm ✓
Common Mistakes
✗ The shop is open by 10 pm every day. → ✓ The shop is open until 10 pm every day.
✗ You must return the books until the end of the week. → ✓ You must return the books by the end of the week.
Frequently Asked Questions
What is the difference between at, on, and in for time expressions?
Use 'at' for precise clock times and fixed points: 'at 9 o'clock', 'at midnight', 'at the weekend'. Use 'on' for specific days and dates: 'on Monday', 'on 15th March', 'on Christmas Day'. Use 'in' for longer periods — months, seasons, years, and centuries: 'in September', 'in winter', 'in 2024', 'in the 20th century'. A simple rule: the more specific the time, the shorter the preposition — from 'in' (widest) through 'on' to 'at' (most precise).
What is the difference between 'for' and 'since'?
'For' is followed by a duration — a length of time: 'for three years', 'for ages', 'for six months'. 'Since' is followed by a starting point — the moment when something began: 'since 2019', 'since Monday', 'since we were children'. Both typically appear with the present perfect when the situation is still true now: 'I have lived here for ten years' / 'I have lived here since 2014'. With past simple, only 'for' is possible: 'I worked there for three years (then left)'.
When do I use 'during' and when do I use 'while'?
'During' is a preposition and must be followed by a noun or noun phrase: 'during the lesson', 'during my holiday', 'during the storm'. 'While' is a conjunction and must be followed by a subject and verb (a clause): 'while I was cooking', 'while she slept', 'while he was on holiday'. A quick test: if you can replace the word with 'in the course of' + noun, use 'during'. If you can replace it with 'at the same time as' + clause, use 'while'.
What is the difference between 'until' and 'by'?
'Until' (or 'till') expresses continuous action up to a point in time — the action keeps going and then stops: 'The library is open until 9 pm', 'I'll wait until you arrive'. 'By' expresses a deadline — the action must be completed at or before a point in time: 'Submit the report by Friday', 'The project must be finished by the end of the month'. Test: if you can rephrase with 'no later than', use 'by'. If you can rephrase with 'and then it stops', use 'until'.