Grammar review
All three words refer back to a person, but each does a different grammatical job.
Who is the subject: it performs the action. The guy who gave you that information was wrong.
Whom is the object: the action happens to it, and it's the correct choice right after a preposition (of, by, with, to): The book was written by whom?
Whose shows possession: The woman whose car I borrowed lives next door.
Quick test: try answering the implied question with he/him. If he fits, use who; if him fits, use whom. In everyday speech, who is increasingly used for both, but whom is still expected after a preposition in more formal or careful writing.
Choose the correct word (who, whom, or whose) to complete each sentence.
1. Could you please tell me (who/whom/whose) that was?✓
2. The men, two of (who/whom/whose) were drunk, walked very slowly.✓
3. The guy (who/whom/whose) computer I was using moved to Brisbane.✓
4. I know several people (who/whom/whose) became very sick from COVID-19.✓
5. She didn't know when the ESL book was written, or by (who/whom/whose).✓
6. I didn't know (who/whom/whose) book that was.✓
7. I don't know (who/whom/whose) gave you that information, but it's inaccurate.✓
8. The police expert was able to determine (who/whom/whose) fingerprints were on the knife.✓
9. These are some of the professors (who/whom/whose) teach at the University of New South Wales.✓
10. I have no idea (who/whom/whose) left the lights on all night.✓
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KEEP PRACTICING — MORE EXERCISES FOR YOU:
Choosing between who and whom is a classic advanced challenge — these exercises tackle other tricky word choices too.