“The woman who called you” and “The woman that called you” both work, but “The woman, who called you, is my sister” and “The woman that called you is my sister” do not mean quite the same thing - and swapping a comma or a relative pronoun in the wrong place can quietly change what a sentence actually says. Relative clauses are the short descriptive clauses that attach extra information to a noun using words like “who,” “which,” “that,” and “whose,” and getting the right pronoun, the right punctuation, and the right clause type is one of the clearest signals of fluency in written and spoken English alike. This guide covers how relative clauses are built, the defining-versus-non-defining distinction that changes meaning, when a relative pronoun can be dropped entirely, and the mistakes that give away a non-native sentence even when the vocabulary is correct.
A relative clause is a clause that describes or identifies a noun, attached directly to it using a relative pronoun: who, whom, whose, which, or that. “The book that I borrowed” - the clause “that I borrowed” tells you exactly which book is being discussed. Relative clauses let English combine two related ideas into a single, efficient sentence instead of two separate ones: “I have a friend” + “My friend lives in Tokyo” becomes “I have a friend who lives in Tokyo.” Without relative clauses, formal and even casual English would sound noticeably choppy, stringing together short, disconnected sentences instead of the smoothly layered descriptions fluent speakers use constantly without thinking about it.
The choice of relative pronoun depends on what it refers to. “Who” and “whom” refer to people (“the man who called,” “the man whom I met”). “Which” refers to things and animals (“the car which broke down”). “That” can refer to either people or things in defining clauses, and is often the more natural, less formal choice in everyday speech (“the man that called,” “the car that broke down”). “Whose” shows possession, for both people and things (“the woman whose car was stolen”).
The second core distinction is defining versus non-defining. A defining (restrictive) clause identifies exactly which noun is being talked about, and removing it would change the sentence's meaning or make it unclear: “The student who studies hardest usually gets the best grades” - without the clause, you don't know which student. A non-defining (non-restrictive) clause adds extra, non-essential information about a noun that is already clearly identified, and it is set off by commas: “My brother, who lives in Tokyo, is visiting next week” - the sentence still clearly refers to your one brother even without the clause. Defining clauses never take commas; non-defining clauses always do.
When the relative pronoun is the object of the clause (someone or something is doing an action to it), it can be dropped in defining clauses: “The movie (that) we watched was boring.” Here, “we” is the subject of “watched,” and “that/movie” is the object - so the pronoun is optional.
When the relative pronoun is the subject of the clause, it can never be dropped: “The man who called you” - “who” is the subject of “called,” so it must stay. Dropping it (“The man called you is my neighbor”) produces a sentence that sounds like a completely different structure, not a relative clause at all.
The relative pronoun can never be dropped in a non-defining clause, regardless of its grammatical role: “My car, which I bought last year, needs new tires” - “which” must stay, since dropping it would eliminate the extra information entirely.
Relative clauses connect two ideas into one fluent sentence, and that fluency shows up as much in rhythm and pausing as in grammar - non-defining clauses are typically set off by a small pause on either side when spoken aloud, mirroring the commas in writing. The Pronounce Checker tool lets you speak full example sentences like “My sister, who lives in Canada, is visiting next month” out loud and get instant feedback on how natural your delivery and pacing sound, not just whether the grammar is technically correct. Practicing the pause pattern alongside the grammar builds genuinely fluent-sounding sentences, not just grammatically accurate ones.
“The man who I met him yesterday” repeats the object with both “who” and “him.” The fix: the relative pronoun already fills that grammatical role - “The man who I met yesterday,” with no extra pronoun needed. Our guide on reported speech in English covers a related case where learners similarly double up on grammatical elements that English only requires once. This particular mistake is extremely common among speakers of languages that allow or require a resumptive pronoun in similar structures, making it worth deliberately unlearning as a general habit rather than correcting sentence by sentence.
“The book, that I read, was excellent” incorrectly adds commas to a defining clause, while “My mother who lives in Rome is a doctor” incorrectly omits them from a non-defining one (assuming the speaker has only one mother). The fix: commas signal “this information is extra, not essential” - use them only when the clause could be removed without losing the sentence's core meaning.
“My father, that taught me to drive, is retiring” incorrectly uses “that” in a non-defining clause. The fix: “that” is reserved for defining clauses only - non-defining clauses always require “who,” “which,” or “whose.” Our guide on conditional sentences in English covers another case where a single small word choice (if vs. when, or here, that vs. which) changes a sentence's grammatical category entirely. Both cases reward the same underlying skill: noticing that a tiny, easy-to-overlook word carries real grammatical weight, rather than treating it as interchangeable with similar-looking alternatives.
A relative clause is a clause that describes or identifies a noun, introduced by a relative pronoun such as who, which, that, or whose: “The book that I borrowed was excellent.”
A defining clause identifies exactly which noun is meant and is essential to the sentence's meaning, with no commas: “The car that broke down was mine.” A non-defining clause adds extra, non-essential information about an already-identified noun, and is always set off by commas: “My car, which is ten years old, broke down.”
You can drop the relative pronoun only in a defining clause where it functions as the object: “The book (that) I read.” You can never drop it when it's the subject of the clause, or in any non-defining clause.
Yes, in defining clauses, “that” can refer to people just as naturally as “who”: “the woman that called” is just as correct as “the woman who called.” “That” cannot be used in non-defining clauses, however, regardless of whether the noun is a person or a thing.
“Who” is used when the relative pronoun is the subject of the clause (“the man who called”), while “whom” is used when it's the object (“the man whom I called”). In everyday spoken English, “who” is increasingly used for both roles, and “whom” is now mostly reserved for formal writing.
Yes, a relative clause should attach immediately after the noun it describes, even if that means it interrupts the main sentence: “The keys that I lost are still missing,” not “The keys are still missing that I lost.”
Relative clauses reward careful attention to two small details - which pronoun fits the noun, and whether commas belong - far more than they reward memorizing a long list of rules. Master the defining/non-defining distinction first, since it governs both the punctuation and the pronoun choice at once. Try speaking today's example sentences out loud using the Pronounce Checker tool to build both the grammar and the natural pausing rhythm together.
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