The three useful forms
1 · `which` / `that` — for things
The policy that was introduced in 2014 reduced sugar consumption by 12 percent.
Defining: tells the reader WHICH policy. No commas, ‘that’ is natural here.
The policy, which was introduced in 2014, reduced sugar consumption by 12 percent.
Non-defining: the policy is already identified by context; the clause adds extra information. Commas required, ‘which’ not ‘that’.
2 · `who` — for people
Students who attend regular tutorials tend to perform better than those who don't.
Defining: tells you WHICH students. No commas. Note the parallel structure with ‘those who’ — natural Band 7+ phrasing.
3 · `whose` — possessive
Cities whose public transport is unreliable see higher car ownership rates.
‘Whose’ works for things AND people in academic English. ‘Cities of which the public transport is unreliable’ is technically correct but reads as stilted.
The comma error that costs you marks
Wrong — defining with commas
The students, who finished early, left the exam.
Right
The students who finished early left the exam.
The wrong version says all the students finished early — because the commas turn the clause into extra info about a known group. The right version restricts: only those students who finished early. A subtle but mark-changing distinction.
Pick the right form + commas
- 1
Which sentence uses relative clauses correctly?
Pick one. You'll see why straight away.
- 2
Cities _____ infrastructure has not kept pace with population growth often suffer from chronic congestion.
Pick one. You'll see why straight away.