On-Phrases — English Grammar Exercises
by mistake, on time, in advance — get the preposition right every time
On-Phrases in English
'On' in fixed phrases frequently describes a current state, an ongoing activity, or a scheduled condition. Learner corpus analysis identifies 'on purpose', 'on time', and 'on business' as among the most error-prone on-phrases at B1–B2 level, partly because related concepts use different prepositions ('by accident' vs 'on purpose') and partly because cognate languages use different prepositions for the same concepts.
Core On-Phrases
on purpose — He didn't break it on purpose — it was an accident.
on fire — Call the fire brigade! The building is on fire!
on sale — Everything in this shop is on sale — 30% off.
on the phone — She's on the phone right now — can she call you back?
on business — He's travelling on business this week.
On Time vs In Time
'On time' = punctual, at the exact scheduled moment. 'In time' = not too late, with enough time to spare. The distinction is covered in detail in the Confusable Pairs subtopic.
Common Mistakes
✗ The house is in fire! → ✓ The house is on fire!
✗ She's travelling in business. → ✓ She's travelling on business.
Frequently Asked Questions
What is the difference between 'on time' and 'in time'?
'On time' means punctual — at the exact scheduled moment: 'The train arrived on time.' 'In time' means not too late — with enough time to spare, often implying it was a close call: 'We arrived in time to catch the last train.' A useful test: if you can replace the phrase with 'punctually', use 'on time'. If you can replace it with 'just in time' or 'before it was too late', use 'in time'.
What is the difference between 'for sale' and 'on sale'?
'For sale' means available to buy at its normal price: 'Is this house for sale?' 'On sale' means available at a reduced, discounted price: 'Winter coats are on sale — 40% off.' In British English, 'on sale' can also simply mean 'available in shops', but in the context of a discount, both British and American English use 'on sale'.
Why do we say 'by mistake' but 'on purpose'?
These are fixed idiomatic expressions with no logical rule — they must be learned as units. 'By mistake' and 'by accident' both describe unintentional actions ('by' here expresses manner or means). 'On purpose' describes intentional action ('on' here is part of a fixed set phrase). Speakers of Russian often confuse these because the Russian phrase «по ошибке» (by mistake) and «намеренно» (on purpose) do not map onto English prepositions predictably.
What is the difference between 'at the end' and 'in the end'?
'At the end' requires 'of' and refers to the final point of a specific, identifiable thing: 'At the end of the film, everyone cried.' 'In the end' means eventually or after a long process of deliberation — no 'of' follows: 'In the end, we decided to stay home.' A simple test: if you can follow it with 'of [something]', use 'at'. If it means 'eventually' or 'after everything', use 'in'.