Relative pronouns ('who', 'whom', 'which', 'that') introduce dependent clauses that modify nouns. 'Who' and 'whom' refer to people, 'which' typically refers to things with a non-restrictive clause, and 'that' typically introduces restrictive clauses. Using the correct relative pronoun ensures clear relationships between ideas.
In sentences with relative clauses, identify the relative pronoun and the noun it modifies. Practice creating relative clauses to provide additional information about nouns.
You already know from studying relative clauses that they are dependent clauses that modify nouns — answering questions like "which one?" or "what kind?" about the noun they follow. Relative pronouns are the words that introduce those clauses and simultaneously hold a grammatical role inside them. Understanding which pronoun to use requires attention to two things: what the antecedent (the noun being modified) refers to, and what grammatical role the pronoun plays inside the clause.
The first distinction is between who/whom (for people) and which/that (for things). "The scientist who discovered the cure" — "who" refers to a person. "The tool that fixed the pipe" — "that" refers to a thing. This is the most basic selection rule. A further distinction within the person category separates who from whom: "who" fills subject roles inside the clause ("the man who called"), while "whom" fills object roles ("the man whom I called"). A quick test — if you can substitute "he" inside the clause and it works, use "who"; if you need "him," use "whom."
The second distinction — between that and which — hinges on whether the clause is restrictive or non-restrictive. A restrictive clause is essential to identifying which noun is meant; without it, the sentence's meaning changes or becomes unclear. "The car that I bought last year is red" uses a restrictive clause — removing it leaves "the car is red," which fails to identify which car is meant. "That" is standard for restrictive clauses. A non-restrictive clause adds supplementary information about a noun already fully identified. "My car, which I bought last year, is red" — you already know which car; the clause is just bonus detail. "Which" is standard for non-restrictive clauses, and these clauses are set off by commas.
Putting it together: relative pronouns are not interchangeable, and commas are not optional decoration. The choice of pronoun signals the antecedent type (person vs. thing) and grammatical role inside the clause; the presence or absence of commas signals whether the clause is essential or supplementary. Both distinctions carry real semantic weight — a misused "that/which" or a missing comma can genuinely change what a sentence means, and readers sensitive to these conventions will notice.