Relative clauses are dependent clauses that modify nouns or noun phrases. They appear in two forms: restrictive and nonrestrictive. This handout explains relative clauses, as well as the differences between restrictive and nonrestrictive constructions.

Relative Pronouns

Identifying the relative pronoun is the first step to understanding relative clauses. In English, there are eight relative pronouns: that, who, whom, whose, which, where, when, and why. Like all pronouns, they take antecedents. An antecedent is simply the noun a pronoun refers to or replaces in a sentence.
Ex. Terry gave her boss a bad review.
 
In this sentence, the pronoun her refers to Terry. Therefore, Terry is its antecedent. When dealing with a relative pronoun, identify its antecedent. Is it a person or a thing? The pronouns who and whom refer to people, while which and that refer to things.
Ex. Person Lars, who loves chocolate, ate too much.
Ex. Thing The chair, which has a broken leg, is wobbly.
 
In the first example, the relative pronoun refers back to the subject (Lars). Since Lars is a person, we use the relative pronoun who. In the second sentence, the chair is a thing, so we use which. Use when for describing time (the day when we got married) and where for places (the house where I was born). Whose shows possession (the man whose house was burglarized).
Subject/Object Position. Assume your antecedent is a person—do you use who or whom?
Ex. The bully whom Jill fears is angry.
 
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, identify the relative pronoun’s role in the dependent clause. Use who for subjects and whom for objects. In our example above, 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.
In the next example, the pronoun who is the subject of the dependent clause.
Ex. Jill fears the bully who is angry.
 
First, identify the relative clause (“who is angry”) and the antecedent it modifies (“bully”). In this case, the antecedent is the direct object of the independent clause, but we still use who because we are only concerned with the relative pronoun’s role in 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. Also use whom when the relative pronoun is the object of a preposition.
Ex. The boys, to whom much was given, were spoiled.
 
Here, the relative pronoun is the object of the preposition to. However, it’s still an object, so we 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.
Ex. Nonrestrictive The well, which is old, ran dry.
Ex. Restrictive The well that is old ran dry.
 
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 the clause 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 specifically the old well. Both constructions are perfectly acceptable; however, the difference in meaning is subtle, so which one you use depends on what you mean.
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.
Ex. Nonrestrictive The bully, whom Jill fears, is angry.
Ex. Restrictive The bully whom Jill fears is angry.
 
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:
Informal or Colloquial Usage of Who. In informal, colloquial usage, and increasingly even in more formal situations, whom is either not used at all or used only in familiar expressions such as to whom it may concern. Before foregoing the use of whom, however, consider your audience. Are you attempting to create a formal or informal tone?  Would whom sound stuffy? Would who sound too casual?
Interrogatives. Pronouns like who and which can also be used to introduce questions. In this case, they are called interrogative pronouns and should not be confused with relative pronouns.
Ex. Who is eating my lunch?
Ex. Which door is she behind?
 
Demonstratives. The pronoun that can also be used as a demonstrative. A demonstrative specifies the object a speaker is referring to, distinguishing it from other objects. Don’t confuse a demonstrative with a relative pronoun.
Ex. Demonstrative Give me that baseball.
Ex. Relative pronoun That is my dog, Ralph.