Relative pronouns introduce a relative clause and connect it to a noun called the antecedent. They point back to a person, thing, or possession while also playing a grammatical role inside the clause, such as subject, object, or possessive. Relative pronouns are important for building Clauses that add essential or extra information to a sentence. They are closely related to Pronouns, especially Subject Pronouns, Object Pronouns, and Interrogative Pronouns.

The choice of relative pronoun depends on the antecedent and the role it plays inside the relative clause. People usually take who for subjects, whom for objects in formal English, and whose for possession. Things usually take which, while that is common in restrictive clauses for both people and things.

Idea	Example
Who marks a person who serves as the subject of the relative clause.	The teacher who explained the lesson was patient.
Whom marks a person who serves as the object of the relative clause in formal English.	The man whom I met was kind.
Whose marks possession for a person or thing.	The author whose novel won was celebrated.
Which marks a thing and often introduces nonrestrictive information.	The car, which was blue, moved slowly.
That marks a restrictive clause for a person or thing.	The dog that barked all night is asleep now.

A relative pronoun agrees with the role it has inside the relative clause, not with the role of the antecedent in the main clause. The verb inside the relative clause agrees with the subject of that clause, so the agreement follows the local subject rather than the earlier noun. This is the same sentence structure often analyzed in Clauses, where the clause has its own internal grammar.

Idea	Example
The pronoun may function as the subject of the relative clause.	The woman who knows the answer is here.
The pronoun may function as the object of the relative clause.	The singer whom we heard was excellent.
The verb agrees with the subject inside the relative clause.	The birds that fly south return in spring.
The antecedent does not control the verb form inside the relative clause.	The book that contains maps is useful.