What a relative clause does
It attaches extra information to a noun without needing a second sentence. "The report, which was published last week, has been widely criticised" and "The report that was published last week has been widely criticised" are doing the same job with different meanings, and the difference between them is punctuation.
The six forms
| Form | For | Example |
|---|---|---|
| who / whom | people | The trainer who marked my essay gave useful feedback. |
| which | things | The chart which shows tourist numbers rises sharply. |
| that | people or things, in a defining clause | The course that helped most was the writing one. |
| whose | possession | The student whose argument was strongest scored highest. |
| where | places, after a preposition | The city where I studied is much quieter. |
| when | time | I still remember the day when I took the test. |
Two corrections to common beliefs here. "That" is not informal. In a defining clause it is standard in both registers, and IELTS uses it freely. What is stylistically marked is using *that* in a non-defining clause, where *which* is preferred.
And where cannot take an object. "The city where I visited" is wrong, because *visit* takes a person as its object and a place as its location. Use "the city which I visited", or reword: "the city I visited".
The comma decides which type
This is the whole distinction, and it is a punctuation mark rather than a grammar rule.
Defining (no commas). The information identifies which noun you mean, so it cannot be removed.
- "The man who lives next door is a doctor." — remove the clause and you do not know which man.
- "The students who failed were given extra classes." — remove it and you do not know which students.
Non-defining (commas both sides). The noun is already identified, so the clause is extra.
- "My neighbour, who is a doctor, works night shifts."
- "Tokyo, which has the largest population, is expensive."
Put a comma in and the meaning changes: "The students, who failed, were given extra classes" now means all of them failed.
Candidates commonly add defining clauses to make sentences complicated. Resist it. A non-defining clause added for decoration is range without accuracy, which is what the grammar criterion is judging.
Where they help in IELTS
Task 1, to attach a detail to a data point:
- "Exports peaked in 2018, a year which coincided with the introduction of the subsidy."
Task 2, to compress a clause and raise range without a long sentence:
- "Policies which reward short-term results encourage exactly the behaviour they were meant to prevent."
- "A system that is cheap to adopt is not necessarily cheap to run."
Speaking, for natural detail:
- "My hometown, which is quite small, has one good library."
Four errors worth memorising
| Wrong | Right | Why |
|---|---|---|
| The person which called me | The person who called me | *which* does not refer to people |
| The city where I visited | The city which I visited | *where* cannot take an object |
| This is the reason which I left | This is the reason why / that I left | *reason* takes *why* or *that* |
| He is the best teacher, that has | He is the best teacher, who has | *which* and *that* do not refer to people |
The fourth is a genuinely common error in otherwise good writing, and it is the one an examiner will notice.
A caveat about complexity
Relative clauses are useful because they let you add information without a fourth sentence. They are not useful because they sound advanced.
An essay where every sentence contains a relative clause reads as laboured, and the coherence criterion penalises exactly that. Two or three well-placed clauses in a 250-word essay is a deliberate range choice. One in every sentence is a tic.
The same applies to omitting the relative pronoun, which is fine in formal English after a comma: "the report, published last week, has been criticised". It is cleaner than "which was published last week" and perfectly acceptable in Task 1 and Task 2.
Practise by expanding one of your own sentences three ways — as two separate sentences, with a defining relative clause, and with a non-defining one — and then use whichever is shortest. That habit produces range and brevity at once, which is what the grammar and coherence criteria are both asking for.
The Grammar Portal has relative-clause exercises, and the IELTS Practice Portal gives you timed writing to try the three versions in.
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