Grammar
Using Who and Whom
The pronoun who is used as a subject.
Who made this mess?
I saw a performer who could do four back flips.
The pronoun whom is used as an object (object of a preposition or direct object).
To whom did you send a letter?
Whom will you ask?
In a question that uses who or whom, change the word order so that the subject comes first.
Then see if the pronoun is used as a subject or an object.