Who and Whom
Who is a subjective pronoun; it should be used to refer to the subject of a sentence. Whom is an objective pronoun; it should be used to refer to the object of a sentence. If you’re stuck, you can try this formula:
if the pronoun can be replaced by he or she, then use who.
if the pronoun can be replaced by him or her, then use whom (you can also look for the preposition).
Whom is the project leader?
If we replaced whom with him, the sentence would be obviously incorrect. As well, there’s no preposition.
Who is the project leader?
He who gossips may soon find the tables turned.
Who did this painting?
To Whom It May Concern:
Note the preposition, to.
With whom should I speak about this problem?
Whom did you meet yesterday?
Even though there’s no preposition, we need to objective case in this sentence; think of it this way: Whom did you meet with yesterday?