"Whose" is possessive (belonging to whom); "who's" is a contraction of "who is" or "who has".
Belonging to whom. Indicates possession or ownership. No apostrophe.
Whose jacket is this? Someone left it in the meeting room.
The student whose essay won the prize is only sixteen years old.
I need to find out whose car is blocking the entrance.
Contraction of "who is" or "who has". Can always be expanded back to its full form.
Who's coming to the party on Saturday? Let me know by Friday.
Do you know who's responsible for this decision?
Who's been using my computer? The settings are all changed.
Common mistake
The apostrophe is the trap: who's = 'who is' or 'who has' (you can always expand it). If expanding it makes the sentence work, use who's. If expanding it makes no sense, use whose. Example: "who's jacket is this?" expanded = "who is jacket is this?" — does not make sense — use "whose". "Whose coming?" — not possessive here — it is "who's coming?" (who is coming).
Trick to remember it
Expansion test: replace the word with 'who is' or 'who has'. If the sentence still makes sense: who's. If not: whose. This trick never fails. "_____ book is this?" — "who is book is this?" — no sense — whose. "_____ there?" — "who is there?" — makes sense — who's.
Quick rule
Whose = possessive (belonging to whom, no apostrophe). Who's = contraction of 'who is' / 'who has'. Test: expand to 'who is' — if it works, use who's.
Did you know?
The same trick works for all possessive/contraction pairs in English: its/it's, your/you're, their/they're. None of the possessives have an apostrophe. It is one of the most common errors even among native English speakers.
Exercise 1 · Pick the right one
_____ responsible for making sure the report is submitted on time?
Exercise 2 · Pick the right one
The author _____ novel won the Booker Prize gave a speech last night.
Exercise 3 · Form the word
Whose = possessive (belonging to whom, no apostrophe). Who's = contraction of 'who is' / 'who has'. Test: expand to 'who is' — if it works, use who's.
The apostrophe is the trap: who's = 'who is' or 'who has' (you can always expand it). If expanding it makes the sentence work, use who's. If expanding it makes no sense, use whose. Example: "who's jacket is this?" expanded = "who is jacket is this?" — does not make sense — use "whose". "Whose coming?" — not possessive here — it is "who's coming?" (who is coming).
Expansion test: replace the word with 'who is' or 'who has'. If the sentence still makes sense: who's. If not: whose. This trick never fails. "_____ book is this?" — "who is book is this?" — no sense — whose. "_____ there?" — "who is there?" — makes sense — who's.
Bryan López
Founder of SpeakUP Academy