Who Versus Whom

I start many letters with "To Whom It May Concern" and usually this is the only time I used the word "whom" in a sentence. When writing an email today, I was stuck as to whether to use who or whom. So, I did some digging and got my answer.

A) Who: when the pronoun acts as the subject of the clause, use who. For example: The prize goes to the runner who collects the most points. [Who does the action of collecting.]

B) Whom: When the pronoun acts as the object of the clause, use whom. For example: The tutor to whom I was assigned was very supportive. [Whom is the object of the preposition to.]

If you can't tell a subject from an object, you can replace who/whom with he/him. If he sounds right, use who; if him is right, use whom. For example: since he did it and not him did it, use who did it; since we give something to him and not to he, use to whom.