Who or whom?

The use of who and whom often causes confusion, so it's a topic worthy of discussion.

Let's begin by looking at the two sentences below:

(1) The man _____ hit you is a bouncer.
(2) The man _____ you hit is a bouncer.

Which of these requires who and which requires whom?

To answer that question, we need to first introduce two definitions:

who is a relative pronoun functioning as subject
whom is a relative pronoun functioning as object

(In grammar, a subject is typically the doer of the action and an object the recipient of the action. So in The dog bit Alice, The dog is the subject of the sentence and Alice is the object.)

Next, we ask the question: Who did the hitting?

The answer to that question will help us find the subject of the verb hit.

In (1), the man did the hitting. We can see this more clearly by simplifying (1) to (1.1):

(1.1) The man hit you; the man is a bouncer

So the man is the (underlying) subject of hit in sentence (1). Consequently, we use who because who is subjective:

(1.2) The man who hit you is a bouncer.

In (2), you did the hitting. Again, it helps to simplify sentence (2) to (2.1):

(2.1) You hit the man; the man is a bouncer.

Sentence (2.1) shows that the man is the (underlying) object of hit. Consequently, we use whom for (2) because whom is objective:

(2.2) The man whom you hit is a bouncer.

The discussion above notwithstanding, the use of whom is considered rather formal. In spoken or informal English, who is preferred over whom in object position. But when there is a preceding preposition, whom is the only choice:

(3) The girl to whom he spoke is Angela.
(4) With whom did you have dinner?