Who is used when you are referring to the subject of a sentence (like he, she, or they), and whom is used when you are referring to the object of a sentence (like him, her, or them). Generally, who does something. Whom has something done to it.
Still confused? Try mentally substituting he or him (or your pronoun of choice) where who or whom should go. If him fits, you want whom (both end in m); if he fits, you want who (both end in a vowel). Ask yourself, “Who is doing what to whom?”
Q: The captain chose teammates (who or whom) he thought played well.
A: The captain chose teammates who he thought played well.
Why: In this case, the who or whom in question has done something: played well. Or, if you use the substitution trick, he or they played well. You wouldn’t say, for example, him played well or them played well. Therefore, you know you want who.
Q: Joe wouldn’t tell John (who or whom) he chose for his fantasy team.
A: Joe wouldn’t tell John whom he chose for his fantasy team.
Why: Here, the who or whom in question is having something (the action of choosing) done to it. Ask yourself, “What did Joe choose?” and substitute. He chose them. You wouldn’t say he chose they. So, you know you want whom.
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