That, Which, Who: Relative Clause Traps in TOEFL Reading and Writing

Halfway through a Write for an Academic Discussion response, you summarize an academic point: "The professor described a method, which was developed in the 1960s, that involves measuring soil density." You hit submit, feel good about the vocabulary, and move on. A rater reads it twice — the commas and the relative pronouns are doing inconsistent work, and the meaning shifts depending on how a reader interprets each clause. Welcome to one of the most underestimated grammar topics on TOEFL iBT 2026: the relative clause.

Relative clauses give English its layering. They let one sentence carry two or three pieces of information without sounding choppy. Used cleanly, they are the difference between "The student. She studied hard. She passed." and "The student who studied hard passed." Used carelessly, they introduce ambiguity, comma errors, and small but cumulative score drops in Writing.

Why This Matters on TOEFL iBT 2026

Read an Academic Passage texts on TOEFL iBT 2026 are saturated with relative clauses. A single academic sentence often nests two or three relative clauses inside its main verb. If you misread which noun a relative clause modifies, you misread the entire sentence — and inference and detail patterns punish that immediately. Sentence-simplification practice is essentially a relative-clause-attachment test: which paraphrase preserves the original layering?

In Writing, relative clauses are how strong responses pack information efficiently. A 130-word Academic Discussion response that uses two well-placed who or which clauses sounds richer than the same response written in choppy short sentences. The catch is that misused that / which commas can change meaning entirely — and raters notice.

In Speaking, spoken relative clauses can be loose without penalty, but excluding them altogether makes your response sound like a list. One controlled relative clause per response signals grammatical range.

In How Do I Solve Vocabulary-in-Context Questions on TOEFL 2026 Reading?, the surrounding clause — often a relative clause — is exactly what gives you the contextual cue. Misreading the attachment is a hidden cause of wrong answers on vocab questions.

The Trap

Five relative-clause issues account for most of the damage on test day.

That vs. which (restrictive vs. non-restrictive). A restrictive clause identifies which one — without it, the sentence doesn't make sense. A non-restrictive clause adds extra information — the sentence still works without it. In formal American English, that is restrictive (no commas) and which is non-restrictive (with commas). "The book that I borrowed is overdue" identifies which book. "The book, which I borrowed last week, is overdue" assumes you already know which book and adds a side comment. Mixing them changes the meaning.

Comma placement. A misplaced comma in a relative clause is more serious than learners realize. "My brother who lives in Berlin is a doctor" (no commas) implies you have multiple brothers and are identifying one. "My brother, who lives in Berlin, is a doctor" (commas) implies you have one brother and are adding extra information. On Reading, these commas are the cue for identification vs. description.

Who vs. whom. Who is the subject of the relative clause; whom is the object. "The professor who teaches the course" — who does the teaching. "The professor whom we met yesterday" — whom is the object of met. Spoken English has largely dropped whom, but it still appears in Reading passages and is sometimes the right answer in sentence-simplification practice under Read an Academic Passage. Writing it incorrectly under time pressure is harmless; reading it incorrectly costs points.

Whose. Whose shows possession and works for both people and things. "The author whose book won the prize" (person). "The theory whose implications surprised everyone" (thing). Many learners replace whose with awkward periphrasis like "the theory of which the implications" — grammatically fine but harder to read and rarely needed.

Missing relative pronoun. In informal English, the object relative pronoun is often dropped: "The book I borrowed is overdue" (instead of "The book that I borrowed"). This is acceptable in Writing, but the subject relative pronoun cannot be dropped: "The book that is on the table" cannot become "The book is on the table" without changing the meaning entirely. Confusing subject vs. object drops is a recurring leak in Write for an Academic Discussion.

Sentence-level vs. clause-level which. Using which to refer to the whole previous sentence — "The professor extended the deadline, which made everyone happy" — is common in spoken English but considered loose in formal Writing. The safer move is to give which a clear noun antecedent: "The professor extended the deadline — a decision that made everyone happy."

Wrong / Better / Why

Where It Shows Up

Read an Academic Passage sentence-simplification patterns. Four answer choices paraphrase a complex original. Three of them subtly reshuffle which clause modifies which noun. The right answer preserves the relative-clause attachment of the original. Reading carefully for that, which, and who is the entire skill.

Write for an Academic Discussion. Summarizing an academic point usually requires a relative clause: "The professor described a method that…" or "The reading mentioned a theory which…". Choosing the right pronoun and the right comma pattern signals comprehension.

Academic Discussion. Strong responses reference the prior students: "Emma's argument, which focuses on cost…" or "Carlos's example, who works at a tech firm…" Wait — who refers to Carlos, not his example. Misattaching the relative pronoun is a small but visible slip.

Take an Interview. Spoken responses tolerate loose which clauses, but a confident whose or who shows range.

Fast Fix

Three quick checks catch nearly every relative-clause error.

First, decide whether the clause is identifying or describing. If you can delete the clause and the main sentence still makes sense without losing essential identification, the clause is describing — use which (or who for people) with commas. If deleting the clause leaves the noun unidentifiable in context, the clause is identifying — use that (or who for people) with no commas.

Second, check who is doing the action of the relative verb. If the noun being described is doing the action, use the subject form: who for people, that or which for things. If the noun being described is receiving the action, you can use the object form (or drop it entirely): whom, that, which, or zero.

Third, avoid stacking two relative clauses on the same noun. If you find yourself writing "The book that I borrowed which is overdue is on the table," break it apart: "The book that I borrowed is on the table, and it is overdue." Two short sentences read better than one overloaded one.

Mini Practice

1. Add or remove commas as needed: The professor who teaches advanced statistics is famous in her field. (Assume the writer has one statistics professor.)
2. Choose that or which: The lecture (that / which) we attended yesterday was the most useful one this semester.
3. Fix the pronoun: The author which wrote the book is giving a lecture next week.
4. Reduce one of the relative clauses to a phrase: The library that is on the corner that I visit often is small.
5. Replace the awkward periphrasis with whose: The article, of which the conclusion was controversial, sparked debate.

Possible improved versions: (1) The professor, who teaches advanced statistics, is famous in her field. (commas because she is the only such professor). (2) The lecture that we attended yesterday was the most useful one this semester. (restrictive). (3) The author who wrote the book is giving a lecture next week. (4) The library on the corner, which I visit often, is small. (5) The article, whose conclusion was controversial, sparked debate.

What to Check Before You Submit

In your final sixty seconds, scan for every that, which, who, whom, and whose in your response. For each one, ask: is this clause identifying which noun I'm talking about (restrictive, no commas) or adding extra information (non-restrictive, commas)? Check that who refers to a person and which refers to a thing — confusing the two is a common low-level slip. Make sure no sentence stacks two relative clauses on the same noun; if one does, break it apart. A clean relative-clause layer makes Writing sound layered and controlled — exactly the texture raters reward.