Who or whom?
Here’s a tricky one: “Who/whom did he see at the party?”
Here’s what you need to know:
Who = subject pronoun
Whom = object pronoun
Whom = object pronoun
So, to get the answer, change the question to a statement: “He did see whom at the party.”
Whom is an object, so that is the answer 
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.Question: “Who/whom ate all the cookies?” Statement: “Who ate all the cookies.”
