For example:
Larry, who loves chocolate, ate too much. Person
The chair, which has a broken leg, is wobbly. Thing

In the first example, the relative pronoun refers back to the subject (Larry). Since Larry is a person, we use the relative pronoun who. In the second sentence, “the chair” is a thing, so we use the relative pronoun which.

Subject/Object

Now, assume your antecedent is a person—do you use who or whom? Let’s look at another example:
The bully whom Jill fears is angry.

“The bully” is our antecedent, and since it is a person, we know we will use either who or whom—but which one? To determine that, we must identify the relative pronoun’s role in the dependent clause (this distinction does not apply to which and that). Use who for subjects and whom for objects.

In our example sentence, the relative pronoun acts as the direct object of the relative clause (whom is the object of Jill’s fear). As a result, we use whom. Let’s look at another example:

Jill fears the bully who is angry.

First, identify the relative clause (“who is angry”) and the antecedent it modifies (“the bully”). In this case, the antecedent is the direct object of the independent clause, but we still use who. This is because we are only concerned with the relative pronoun’s role in the dependent clause! In our example, the pronoun who is the subject of the dependent clause. You can test this by replacing the pronoun with a noun that makes sense (e.g. John is angry). Because the relative pronoun is the subject, we use who.

Object of a Preposition

You now know to use whom with direct and indirect objects, but you also use it when the relative pronoun is the object of a preposition. For example:

The boys, to whom much was given, were spoiled.

In this case, the relative pronoun is the object of the preposition to. However, it is still an object, so we still use whom.

Restrictive/Nonrestrictive

Now assume your antecedent is a thing—do you use which or that? The answer depends on whether the relative clause is restrictive or nonrestrictive. Let’s look at a few examples:

The well, which is old, ran dry. Nonrestrictive

The well that is old ran dry. Restrictive

The relative clauses in both sentences are underlined. The first is nonrestrictive because the relative clause can be removed without altering the meaning of the sentence (The well ran dry). Use which with nonrestrictive clauses and set them off with commas.

Using that in the second sentence makes the relative clause restrictive, meaning it cannot be removed without altering the meaning of the sentence. Therefore, there are no commas around the relative clause in the second example. Use that with restrictive clauses and do not set them off with commas.

Think of it like this: the subject of the first sentence is “The well,” and the relative clause is extra information modifying it. The subject of the second sentence is “The well that is old.” Not just any well, but the specific old well. Both constructions are perfectly acceptable; however, the difference in meaning is subtle, so which one you use depends on what you are trying to convey.

Restrictive/Nonrestrictive and Who/Whom

Restrictive and nonrestrictive constructions can also be used with who and whom. The only way to indicate that a who/whom clause is nonrestrictive is with commas. For example:

The bully, whom Jill fears, is angry. Nonrestrictive

The bully whom Jill fears is angry. Restrictive

Once again, in the first sentence, the subject is “The bully,” and Jill fears him. In the second sentence, the subject is the specific “bully whom Jill fears.”

Exceptions
There are several common points of confusion when dealing with pronouns:

1. Interrogatives: Pronouns like who and which can also be used to introduce questions. In this use they are called interrogative pronouns and should not be confused with their relative uses.

Example: Who is eating my lunch?
Which door is she behind?

2. Demonstratives: The pronoun that can also be used as a demonstrative. A demonstrative specifies what object a speaker is referring to, distinguishing it from other objects. Do not confuse this with its use as a relative pronoun.

Example: Give me that baseball.
That is my dog, Ralph.