Where it appears in the exam
What is it?
Non-defining relative clauses add extra information about a person, thing or idea that is already identified: they sit between commas and do not define, they only comment. English marks this far more clearly than learners expect; for many, a simple pause or comma feels like enough, but it is not. In the exam, the key forms are who, which and whose; on top of that, which can refer to a whole preceding sentence.
Why it matters in the exam
In B2 First this competency appears above all in Part 2, where you have to produce the exact word with no options, and in Part 4, where you must rebuild the sentence keeping the same meaning. If you fail here, you do not just lose the relative point: you also show that you cannot tell essential information apart from an added comment, one of the fine traps of the exam.
The cognitive trap
"Your instinct reaches for one all-purpose relative ('that') for everything"
This is overgeneralization: your brain has learned that a single relative word can join almost any two clauses, so it stops checking what changed. But the comma changes the rules: once the information becomes a non-defining comment, the all-purpose relative is no longer allowed, and the form must agree with person, thing/idea or possession.
"I spoke to a patient, that had suddenly become worse." → "I spoke to a patient, who had suddenly become worse."
A non-defining clause with commas needs the right relative: who for people, which for things or ideas and whose for possession. That does not work here.
Recognition pattern
In the exam, look for the key signal first. The answer follows.
Signals that give it away
"My brother, who lives in Leeds, is visiting next week."
"The athlete, whose diet is extremely strict, won the race."
"She missed the deadline, which surprised everyone."
"Mr Lewis, who taught me chemistry, has retired."
"The film, which was far too long, still won an award."
The errors that Cambridge exploits
"A neighbour of mine, his job involves long hours at a desk, decided to walk to work."
Cambridge penalises this because his cannot join two clauses as a relative. The grammatical connector that whose provides is missing.
"A neighbour of mine, whose job involves long hours at a desk, decided to walk to work."
It is correct because the information about the job only adds detail and possession is expressed with whose.
"It grows through habits that seem small at first, that is encouraging for anyone who wants to begin today."
Cambridge penalises this because that is not used in non-defining relative clauses, especially after a comma.
"It grows through habits that seem small at first, which is encouraging for anyone who wants to begin today."
Which picks up the whole preceding idea, not just the word habits.
"I spoke to a patient, which had suddenly become worse."
Cambridge penalises this because which is not used for a specific person in this context.
"I spoke to a patient, who had suddenly become worse."
Who is the correct relative for a person in a non-defining clause.
"The report, was published last week, caused a lot of debate."
Cambridge penalises this because you have left a clause with no relative. In English a comma is not enough: you need to join it with which.
"The report, which was published last week, caused a lot of debate."
The main clause already identifies the report; the rest is additional information between commas.
Why your brain gets it wrong
The learner's short circuit
Analyse the trap by exam format
My cousin, ______ parents run a small cafe, has decided to study nutrition.
Your instinct reaches for a plain possessive ('their parents', 'his parents') and tries to solve the gap with it. But the gap does not ask for loose possession: it asks for a word that connects the two parts of the sentence and expresses possession at the same time.
parents run
There are commas and right after the gap comes a plural noun: parents.
→ whose
In Open Cloze the options do not help you
In Part 2, Cambridge forces you to tell a plain possessive apart from a possessive relative. The problem is not vocabulary: it is seeing that the gap connects two clauses and that possession is part of that connection.
Strategy
If you see comma + noun immediately after the gap and the meaning would be 'whose', try whose first. Do not put his/her/their if the sentence needs a relative link.
I spoke to a patient, ______ had suddenly become worse.
The instinct is to reach for an all-purpose that and use it by reflex in any relative clause. Here it fails because the comma changes the structure: you are no longer defining which patient, you are only adding information about them, and that is not used after a comma.
a patient,
The comma before the gap signals added information, not identifying information.
→ who
After a comma, that is ruled out
In Part 2, a single gap after a comma decides the relative. The trap is that that seems to be the natural relative, but the comma forces a non-defining relative clause with who (person), which (thing/idea) or whose (possession).
Strategy
When the gap sits right after a comma, drop that from your mental options and choose between who, which, whose based on person, thing/idea or possession.
Non-defining relative clauses is 1 of 82
The exam tests 82 grammar competencies across 19 families. Mastering one is the first step. Automating all 82 is passing.
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