In, After, Ago — English Grammar Exercises
at, on, in, for, since, during, until, by — every time preposition in one place
In, After, Ago — Intervals and Distance in Time
Three expressions handle time measured as a gap from the present moment. Each has a distinct grammar and a distinct meaning, and mixing them — particularly using after where in is required — is one of the most reliable markers of L1 interference at B1–B2 level, especially for Russian and other Slavic language speakers.
In + Time Period — From Now, Looking Forward
In + a duration means 'after that amount of time from now'. It points to a future moment:
The train leaves in twenty minutes — we'd better hurry.
I'll call you in a few days.
Ago — From Now, Looking Backward
Ago counts a distance back from the present. It always follows the time expression and is used with past simple (not present perfect):
She graduated from university two years ago.
I started this job six months ago.
After — Sequence, Not Interval from Now
After in time expressions describes a sequence: 'after the meeting', 'after dinner', 'after arriving'. It does not express a fixed interval from the present moment. Using 'after five minutes' to mean 'in five minutes' is a direct translation error for many learners:
✓ After the meeting, we went for coffee. (sequence — correct use of 'after')
Common Mistakes
✗ The bus arrives after fifteen minutes. → ✓ The bus arrives in fifteen minutes.
✗ We met before three years at a conference. → ✓ We met three years ago at a conference.
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'.