Fixed Expressions and Mixed Practice — English Grammar Exercises
at, on, in, for, since, during, until, by — every time preposition in one place
Fixed Time Expressions and Mixed Practice
Beyond the systematic rules, English contains a set of time expressions that must be learned as fixed phrases because their prepositions do not follow the general at/on/in scale. The most tested pairs are on time / in time and at the end / in the end. According to exam statistics from Cambridge Assessment, fixed time expression errors appear in approximately 22% of B2 writing exam scripts, most commonly the in/at the end confusion.
On Time vs In Time
On time means punctual — arriving or happening exactly at the scheduled moment. In time means not too late — arriving before a deadline or before something happens:
We arrived just in time to catch the train. (= not too late, before it left)
The meeting started on time. (= at the planned time)
She got there in time for the opening speech. (= before it began)
At the End vs In the End
At the end refers to the final point of a specific, identifiable period or sequence — it must be followed by 'of + noun'. In the end means eventually or after a long process of deliberation — it does not require a following noun:
At the end of the day, I was exhausted. (final point of the day)
In the end, we decided to cancel the trip. (eventually, after deliberation)
I didn't enjoy the course at first, but in the end I learned a lot. (eventually)
Other Key Fixed Expressions
At first, I was nervous. (= in the beginning, when I started)
I'll be back in ten minutes. (interval from now)
We arrived just at the end of the film. (the specific final point)
Common Mistakes
✗ I didn't enjoy it, but at the end I learned a lot. → ✓ ...but in the end I learned a lot.
✗ I've been working here for 2024. → ✓ I've been working here since 2024.
✗ I was born in 15th March, 1990. → ✓ I was born on 15th March, 1990.
✗ I started school on September that year. → ✓ I started school in September that year.
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'.