IELTS grammar: Relative Clauses

Relative clauses matter for scoring because they are one of the most reliable, learnable ways to raise Grammatical Range without changing the actual content of your argument — two short, choppy sentences can become one longer, more sophisticated sentence purely through a relative pronoun, with no new vocabulary or ideas required. The specific errors that recur across scripts fall into two categories: choosing the wrong relative pronoun for the noun type (using "which" for a person, or "who" for a thing), and mishandling the comma rule around non-defining clauses, which is not a minor punctuation slip but a change that alters what the sentence actually claims.

A relative clause adds information about a noun using a relative pronoun: "who" or "whom" for people, "which" for things, "that" for either type in restrictive (defining) clauses only, and "whose" for possession regardless of whether the possessor is a person or a thing. "The policy, which was introduced in 2018, has had mixed results" adds background detail about "the policy" inside the same sentence rather than starting a new one. INCORRECT: "The report who was published last month contains useful data." CORRECT: "The report which was published last month contains useful data," or equally correctly, "The report that was published last month." "Who" is reserved for people; a report is a thing, so it needs "which" or "that."

A defining (restrictive) relative clause identifies exactly which member of a category you mean, and removing it would change the sentence's meaning, so it takes no commas: "Students who study regularly tend to perform better on exams." Removing "who study regularly" changes the claim entirely — the sentence is not about all students, only about the specific subset who study regularly. In this restrictive type, "that" is often the more natural, slightly less formal choice for things, and is fully acceptable in IELTS writing: "The policy that was introduced last year has been controversial" is correct.

A non-defining (non-restrictive) relative clause adds extra, non-essential information about a noun that is already fully identified without the clause, and it requires commas on both sides (or one side, if it ends the sentence): "The Amazon rainforest, which produces roughly twenty percent of the world's oxygen, is under increasing threat from deforestation." Removing "which produces roughly twenty percent of the world's oxygen" does not change which forest is being discussed — "the Amazon rainforest" was already a fully specific, identified noun before the clause was added; the clause only adds a supporting fact. Critically, "that" cannot be used in non-defining clauses: INCORRECT "The Amazon rainforest, that produces..." is ungrammatical; only "which" works here.

The comma rule is not decorative — removing or adding commas around a relative clause genuinely changes what a sentence claims. Compare: "Employees who arrived late were penalised" (defining — only the specific employees who arrived late were penalised; other employees were not) versus "Employees, who arrived late, were penalised" (non-defining — this now claims all the employees arrived late, and all of them were penalised, since the clause is presented as extra information about a group already fully identified as "employees"). This is one of the few grammar points on this list where a punctuation error is also, simultaneously, a meaning error, which is exactly why examiners weight it more heavily than a typical comma slip.