Relative clauses

A defining relative clause tells the reader which person or thing you mean — the information is essential, so the clause takes no commas. Remove it and the sentence changes meaning or stops making sense.

In defining clauses you can use who or that for people, and which or that for things. That is slightly more informal and extremely common in speech.

• The bus that stops outside the library goes to the airport.(identifies which bus — no commas)
• Students who submit late lose ten per cent.(defines which students)
• The mechanic, that fixed my car, was brilliant.(defining clause takes no commas, and that never follows a comma → The mechanic that fixed my car was brilliant.)

A non-defining relative clause adds extra information about a noun that is already fully identified — a name, your only sister, a unique landmark. The clause sits between commas, and if you delete it the sentence still identifies the same person or thing.

Two hard rules follow: non-defining clauses never use that (only who for people, which for things), and the relative pronoun can never be omitted. In speech, the commas surface as short pauses either side of the clause.

• My grandmother, who turns ninety in June, still tends her garden.(extra information between commas)
• The Bosphorus, which splits Istanbul in two, is busiest at dawn.
• Salvador, that hosts Brazil's biggest carnival, sits on the coast.(non-defining clauses never use that → Salvador, which hosts…)

Who refers to people, which refers to things and animals, and that can replace either — but only in defining clauses. Whose shows possession and works for both people and things. Where stands in for 'in/at which' after places, and when after time nouns like day, year, and moment.

For organisations and companies, careful writing prefers which or that ('the firm that hired me'), even though everyday speech sometimes drifts to who because a company is made of people.

• The volunteers who organised the cleanup collected forty bags of litter.
• An author whose books sell millions rarely needs a day job.(whose = possession)
• The start-up which office we visited has just been acquired.(which cannot show possession → The start-up whose office we visited…)

In a defining clause, if the relative pronoun is the object of its clause — a different subject follows it — you can drop the pronoun entirely: 'the film (that) we watched'. This reduced form is the natural rhythm of fluent English, and examiners hear it as native-like economy.

You can never drop a subject pronoun (the clause would lose its subject), and you can never drop anything from a non-defining clause. Quick test: if the word straight after the pronoun is a verb, the pronoun is the subject and must stay.

• The advice she gave me saved the interview.(object pronoun dropped — that/which is optional here)
• The neighbour lent me his drill has moved away.(a subject pronoun cannot drop → The neighbour who lent me his drill has moved away.)
• The hotel we stayed at in Hanoi had no lift.(pronoun dropped, preposition stays at the end)

The relative pronoun does not just introduce the clause — it replaces the noun inside it. In 'the photos which she took', which already is the photos, so adding a second pronoun ('which she took them') repeats the noun twice. That extra word is called a resumptive pronoun, and it is one of the most persistent errors for speakers of Arabic, Farsi, Chinese, and Bengali, whose relative clauses genuinely keep such a pronoun.

The fix is a proofreading habit: every time you write a relative clause, scan it for a second pronoun — it, them, there, him, her — that refers back to the head noun. If you find one, delete it; the clause is already complete without it.

• The neighbourhood where I grew up there has changed completely.(there repeats where → The neighbourhood where I grew up has changed completely.)
• She showed me the photos which she took them in Lagos.(them repeats which → the photos which she took in Lagos)
• The scholarship that I applied for arrived just in time.(for stays, but no extra it — the clause is complete)