You’ve been waiting your whole life for this one. “Who” is a subject pronoun. “Whom” is an object pronoun.
You would say, “Robert, who saved my cat, is my hero.” Robert is the subject of this sentence, so he is replaced by “who”.
BUT you would say, “My cat, whom Robert saved, is now safe.” My cat is the object of this sentence (the action, saving, is done to him), so he is replaced by “whom.”
So, famously, the correct title of the pop song would be “Somebody Whom I Used to Know” as the “whom” here is the person being known, and NOT the subject of the phrase.
Here’s a mnemonic, care of Grammar Girl online (don’t mock – it’s one of the top podcasts on iTunes) – if the answer of the question would be “him,” use “whom” which also ends in – M. If the answer to the question is “he”, use “who” For instance, “Who stole my cheese?” “HE did.” “Whom will he select next?” “HIM”. Ta da.
Another good sentence to remember: Who saved whom? (that bumper sticker with a paw print on it gets this wrong… totally wrong…)
* Who is the subject of the sentence (the savior).
* Whom is the object of the sentence (the person being saved).
Who vs. Whom Exercises
Fill in the blank with either ‘who’ or ‘whom’
The conductor of the train, ________________ I gave my ticket to, smiled blankly at me.
Napoleon, _________________ inspired fear in his contemporary Europeans, sternly looked over the battlefield.
This is the governess ___________________ you met in Madrid in the ‘80s.
The burglar, _________________ saw us as he broke into our neighbor’s house, is still at large.
The officer to __________________ we gave our report seemed suspicious.