The simplest way to decide whether to use who or whom is to see whether you can replace it with the words he or she.
If your sentence doesn’t make sense when you substitute “he/she”, use whom. For example:
Right: Whom are you referring to?
Wrong: She are you referring to?
Right: To whom did the prize go?
Wrong: To he did the prize go?
If your sentence still makes sense when you replace the word with he/she, use who. For example:
Right: Who went to the shops?
Wrong: He went to the shops
Right: Who is going to make dinner?
Wrong: Is she going to make dinner?
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