Confusable Pairs — English Grammar Exercises
by mistake, on time, in advance — get the preposition right every time
Confusable Prepositional Phrase Pairs
Even learners who have memorised individual prepositional phrases often struggle when two similar phrases must be distinguished in context. The English Profile Programme identifies 'on time' vs 'in time' and 'for sale' vs 'on sale' as two of the ten most error-prone lexical pairs at B1–B2 level. The confusion arises not from ignorance of the phrases but from misreading contextual clues — the ability to distinguish them is a genuine B2 reading and writing skill.
On Time vs In Time
'On time' = punctual, at the exact scheduled moment. No suggestion of a close call.
'In time' = not too late — arrived before a deadline or opportunity closed, often implying it was nearly missed.
We arrived just in time to catch the last train. (= barely made it; one minute later and we'd have missed it)
Context test: if 'exactly', 'punctually', or a specific scheduled time appears, use 'on time'. If 'just', 'barely', 'almost missed', or 'before it was too late' appears, use 'in time'.
For Sale vs On Sale
'For sale' = listed as available to buy, at the asking price. No implication of discount.
'On sale' = available at a reduced, discounted price.
Winter coats are on sale — 40% off. (= discounted price)
At the End vs In the End
'At the end of' = the final point of a specific, identifiable thing. Always followed by 'of'.
'In the end' = eventually, after deliberation or a long process. Never followed by 'of'.
In the end, we decided to stay home. (= eventually, after considering options)
Common Mistakes
✗ These jackets are for sale — 50% off! → ✓ These jackets are on sale.
✗ At the end, after thinking all night, she accepted the offer. → ✓ In the end, she accepted the offer.
✗ In the end of the film, everyone cried. → ✓ At the end of the film, everyone cried.
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'.