In, On, At — Fixed Expressions

Learn the high-frequency fixed expressions with in, on, and at that cannot be predicted from the general spatial rule.

Fixed Expressions with In, On, and At

Beyond the spatial logic, English has dozens of fixed prepositional expressions that must be memorised. The British National Corpus shows that fixed-expression preposition errors are the most common preposition mistake type in intermediate learner writing — learners apply spatial logic to expressions that have become idiomatic. The most error-prone pairs are 'on fire' vs 'in fire', 'on sale' vs 'at sale', and 'in danger' vs 'on danger'.

Fixed Expressions with 'On'

on fire — burning
on sale — available at a reduced price
on the phone — engaged in a call
on the coast / on a river — along a geographical edge
on the second floor — floors are always surfaces

The building was on fire. (not 'in')
All winter coats are on sale this week. (not 'at')
She's been on the phone for an hour. (not 'in')

Fixed Expressions with 'In'

in danger — at risk
in trouble — having serious problems
in the queue — waiting in a line
in bed — sleeping or resting

The whole village is in danger of flooding. (not 'on')
There were fifty people in the queue. (not 'on')

Fixed Expressions with 'At'

at a distance — from a distance
at work / at home / at school — at a functional location

Common Mistakes

✗ The warehouse was in fire. → ✓ The warehouse was on fire.
✗ Nobody was on danger. → ✓ Nobody was in danger.
✗ She arrived in the airport. → ✓ She arrived at the airport.
✗ She called me at her phone. → ✓ She called me on her phone.