A relative clause modifies a noun by providing additional information, introduced by a relative pronoun (who, whom, which, that, whose). Restrictive relative clauses narrow the identity of the noun and are essential to the sentence's meaning (The student who studied passed), while nonrestrictive clauses add parenthetical detail and are set off by commas (Maria, who studied all night, passed easily). The distinction between restrictive and nonrestrictive determines both punctuation and pronoun choice — "that" introduces restrictive clauses, while "which" typically introduces nonrestrictive ones.
Take sentences with relative clauses and test whether removing the clause changes which person or thing is being identified. If removing it makes the noun ambiguous, the clause is restrictive (no commas); if the noun is already clear, the clause is nonrestrictive (commas required).
You've studied noun phrases — the way nouns expand with determiners, adjectives, and other modifiers to form a full nominal unit. And you've studied dependent clauses — subordinate structures that cannot stand alone as sentences. A relative clause sits at the intersection of these two concepts: it is a dependent clause that functions as a modifier inside a noun phrase, expanding a noun by providing information about it. The relative clause "who studied all night" in "the student who studied all night" is a dependent clause doing the job of an adjective, modifying *student*.
The most important distinction in relative clauses is restrictive versus nonrestrictive. A restrictive relative clause narrows the reference of the noun it modifies — it restricts which member of a class is being identified. "The students *who passed the exam* received certificates." The clause *who passed the exam* is doing real identifying work: without it, we wouldn't know which students. Remove it and the sentence becomes less precise, possibly false. Because the clause is essential to meaning, no commas separate it from the noun. A nonrestrictive clause, by contrast, adds supplementary information about a noun already sufficiently identified. "Maria, *who passed the exam*, received a certificate." Maria is already identified by name; the clause *who passed the exam* adds a fact but does not change which person we're talking about. Remove it and the sentence remains fully accurate. Commas signal this parenthetical status.
The relative pronouns do different jobs in these two clause types. *That* introduces restrictive clauses only: "The book *that* I recommended is sold out." *Which* introduces nonrestrictive clauses (in formal American English), always set off by commas: "The book, *which* I recommended last spring, is sold out." *Who* and *whom* refer to people in either type, with commas determining the type. *Whose* indicates possession: "The author *whose* book won the prize." This that/which distinction is a convention rather than a logical necessity — British English treats *which* as acceptable in restrictive clauses — but in formal American academic and professional writing, maintaining the distinction adds clarity and signals command of the conventions.
The practical skill is asking a two-part question about any relative clause: (1) Is the noun already uniquely identified, or does the clause identify it? (2) Would removing the clause change which entity the noun refers to? If the clause is doing identifying work — if without it the noun would refer to a different or ambiguous set — it is restrictive and takes no commas. If the noun is already pinned down and the clause adds a bonus fact, it is nonrestrictive and gets commas. Getting this right matters not only for punctuation but for meaning: "The students who failed received extra help" (only those who failed) says something very different from "The students, who failed, received extra help" (all the students failed).