Short sentences are safe; joined sentences are B2. Relative clauses are the main joining tool of written English, and they come in two kinds that look almost identical but obey different rules. The difference is worth learning properly, because it is carried by a comma — and a comma can change a fact.
What you can do after this lesson
You can fold two sentences into one with who, which, that, whose, where, decide whether the clause needs commas, and hear what the commas do to the meaning.
The lesson
Defining clauses — essential information, no commas. The clause identifies which one you mean; remove it and the sentence loses its meaning:
The students who submitted early received feedback first.
(Only those students — the clause defines the group.) In defining clauses you may use that instead of who/which, and you may drop the pronoun entirely when it is the object: the report (that) I wrote.
Non-defining clauses — extra information, commas required. The noun is already fully identified; the clause just adds a fact:
Dr. Kila, who joined the faculty in 2024, teaches the research methods unit.
Rules that follow from the commas: no that, and the pronoun can never be dropped. Think of the commas as brackets around a whisper.
The full pronoun kit: who (people), which (things), whose (possession — people and things: a company whose records were lost), where (places), when (times). Formal writing also uses preposition + which/whom: the committee to which the report was sent.
The comma test: read the sentence without the clause. Still true and still specific? → non-defining, commas. Suddenly vague or wrong? → defining, no commas.
Examples
- Defining: Candidates whose forms are incomplete will not be shortlisted.
- Non-defining: The Sepik, which is one of the world's great river systems, floods every wet season.
- Meaning flip: My brother who lives in Kokopo is a nurse (I have several brothers) vs My brother, who lives in Kokopo, is a nurse (I have one).
- Whole-clause which: The seminar finished early, which surprised everyone. (Here which refers to the entire previous idea — non-defining only.)
Common mistakes
- ✗ The lecturer which teaches statistics… → ✓ The lecturer who… (which is for things.)
- ✗ My hometown, that is near the coast, … → ✓ …, which is near the coast, … (that is banned after a comma.)
- ✗ The office where I applied to… → ✓ the office where I applied or the office that I applied to. (Where already contains the preposition — don't pay twice.)
- Comma-blindness in exams: writing a defining clause with commas tells the marker every student submitted early. Punctuation is content at B2.
Self-check — what can I do now?
- Join, deciding on commas: The library extension opened in March. It doubled the study space. (…, which doubled the study space.)
- Explain aloud the difference between the staff who attended the training and the staff, who attended the training. Which one means everyone attended?
- Write one true sentence about your town using whose or where — then apply the comma test to your own sentence.