Until and By
Master the contrast: 'until' means a continuous action or state that reaches a cutoff point; 'by' marks a deadline — something must be completed at or before a specific time.
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.