Relative Clauses in OET Writing: Who, Which, and That

Relative clauses in OET writing cause more small, repeated Language errors than almost any other structure, because the grammar itself is straightforward but the punctuation decision underneath it isn’t. Most candidates know that “who” refers to people and “which” refers to things. Far fewer can reliably say, mid-sentence, whether the clause they’ve just written needs commas around it — and getting that wrong doesn’t just look untidy. In a referral or discharge letter, a missing comma can quietly change what the sentence claims about the patient.

Watch the full breakdown: Grammar Quick Fix #4 works through the who/which/that decision using real OET case-note sentences.

Why do relative clauses cause so many errors in OET letters?

Relative clauses appear constantly in clinical writing because case notes are dense with people and conditions that need identifying — “the patient who presented with chest pain,” “the medication which was discontinued,” “her GP, who had referred her initially.” A typical OET letter task involves several patients, family members, medications, and past events, and relative clauses are the standard tool for attaching a description to any of them without starting a new sentence.

The trouble is that English marks the difference between two kinds of relative clause with punctuation alone, not with a different verb form or word order, so the sentence often looks correct even when the comma decision is wrong. A candidate under time pressure writes “Mr Hassan, who has type 2 diabetes was admitted” — one comma, not two — and moves on, because nothing about the sentence looks obviously broken. It reads fluently. It just isn’t correctly punctuated, and inconsistent or missing relative-clause commas are common enough across L1 groups that examiners recognise the pattern immediately.

What’s the actual difference between defining and non-defining relative clauses?

The test is simple, and it’s the same test in every case: remove the clause and see if the sentence still identifies the same person or thing.

Defining relative clauses tell the reader which person or item you mean. Without the clause, the reader can’t tell who or what you’re referring to. These take no commas.

• The patient who developed the rash was reviewed by dermatology. Remove the clause: “The patient was reviewed by dermatology.” Which patient? Unknown. The clause is doing essential identifying work, so no commas.
• The medication which caused the reaction has been discontinued. Remove the clause: “The medication has been discontinued.” Which medication? Unknown. Defining, no commas.

Non-defining relative clauses add extra information about someone or something the reader can already identify from the rest of the sentence. Remove the clause, and the core meaning survives. These require a comma before the clause and, if the sentence continues, a comma after it.

• Mr Hassan, who is 54 years old, was admitted with acute abdominal pain. Remove the clause: “Mr Hassan was admitted with acute abdominal pain.” Still identifies Mr Hassan completely. The age is extra detail, so it’s fenced off with commas.
• The metformin, which she has taken for three years, was continued at the same dose. Remove the clause: “The metformin was continued at the same dose.” Still complete. Non-defining, commas required.

Once you have the test, the punctuation decision stops being a guess and becomes something you can check on every sentence in under five seconds.

Which relative pronoun: who, which, or that?

The pronoun choice depends on what the clause refers to, and then on whether the clause is defining or non-defining.

The rule that trips up the most candidates: “that” is only ever correct inside a defining clause, and it should never appear immediately after a comma. “Mr Chen, that has type 1 diabetes, was reviewed” is wrong twice over — “that” can’t follow a comma at all, and clinical register avoids “that” for people regardless of clause type. The correct version is “Mr Chen, who has type 1 diabetes, was reviewed.”

The second common slip is using “that” for a person even in a defining clause. It’s grammatically tolerated in everyday spoken English — “the doctor that examined her” is something a native speaker might say without thinking twice — but professional and clinical writing consistently prefers “who” for people. “The doctor who examined her” is the form examiners expect under Genre and Style, and it costs nothing to use it correctly from the start.

When a missing comma actually changes what you’re claiming

This is the part that turns a punctuation habit into a genuine scoring risk. Compare these two sentences:

“Her son, who lives locally, visits daily.”

“Her son who lives locally visits daily.”

The first sentence has one son, and you’re adding the extra fact that he lives locally. The second sentence, read strictly, implies she has more than one son — the clause is defining which son you mean, distinguishing him from a son who presumably lives elsewhere. If the patient only has one son, the second version is not just poorly punctuated; it makes an inaccurate implicit claim about her family structure, and a careful examiner reading for Content accuracy will notice the mismatch between what the case notes state and what the letter implies.

The same risk shows up around medical history. “The patient’s daughter who has asthma collected the prescription” (defining — implies multiple daughters, distinguishing the one with asthma) reads differently from “The patient’s daughter, who has asthma, collected the prescription” (non-defining — one daughter, with an added fact). If the case notes give you only one daughter, only the second form is accurate. This is why relative-clause punctuation sits under the Language criterion for form but can bleed into Content and Clarity when the ambiguity changes the clinical picture the reader takes away from the letter.

Worked examples from real OET letter contexts

Notice the last row: the “correct” fix depends on context, not on a fixed rule about which sentences should have commas. If the letter has only just introduced one patient, the clause is almost always non-defining. If several patients or family members are already in play and the clause is distinguishing one from another, it’s defining. Reading the two sentences it would produce — one with commas, one without — and asking which one accurately describes your case notes is the fastest way to check.

A quick self-check before you submit

Go through your draft and mark every “who,” “which,” and “that.” For each one, ask two questions in order: first, does the clause refer to a person (who/whom) or a thing (which)? Second, would removing the clause still let the reader identify who or what you mean? If yes, it’s non-defining — add both commas. If no, it’s defining — no commas, and “that” becomes an option for things (never for people, in professional register).

This check pairs naturally with a wider punctuation pass — our punctuation in OET writing guide covers comma rules beyond relative clauses, and the 8 grammar errors that cost Language marks lists relative-clause punctuation alongside the other patterns examiners flag most often. The Language criterion breakdown explains exactly how grammar accuracy is weighted at each band. If you want a fast first check on a specific letter, our free OET Grammar Checker will flag obvious comma and pronoun errors, though it won’t catch every case where the ambiguity depends on how many people your case notes actually introduce.

Why this is worth fixing properly, not just patching

Relative clauses are one of those structures candidates use dozens of times per letter without noticing, which means an inconsistent habit — sometimes punctuating correctly, sometimes not — shows up as a repeated pattern across the whole script rather than a single slip. Examiners read for patterns, not isolated mistakes, and a script with five or six relative clauses punctuated five or six different ways reads as genuinely uncertain grammar, even if the underlying sentences are otherwise well-formed. Our OET writing correction service checks exactly this kind of structural consistency across a full letter, alongside the other five criteria, so patterns like this get caught before they cost you a band.