When to use who vs. whom?

Answer 1

If you can answer the question with "he," "she," or "they," use who.

If you can answer the question with "him," "her," or "them," use whom.

For example:

Who took the cookies? He did.
To whom are you speaking? I'm speaking to her.

Statements work the same way as questions:

Give the gift to whomever you please. (Give the gift to him.)
Give the gift to whoever seems to want it most. (She seems to want it most.)
Whoever shows up first will win the prize. (He shows up first.)

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Answer 2

Google "Who vs. whom"

Various useful links will show.
Each will help you out in answering your question.

Answer 3

you use who in sentences like who callled, who did that, who sang that, and you use whom when in sentences like whom do want to talk to, from whom did get the gift. whom=passive who=active