Who, Which, or That? 8 Rules for English Relative Clauses That Finally Make Sense

In Spanish, you only need one word to attach extra information to a noun: «que». La persona que conozco. El libro que leí. Simple.

English makes you choose. Is it who or which? Do you need a comma? Can you say that? Can you just… leave the word out entirely?

These little clauses — called relative clauses — are one of the most common sources of small, repeated mistakes in intermediate and advanced English. The good news: once you learn the eight rules below, the choice stops being a guess and becomes automatic.

1. Start with the basic split: who for people, which for things

“Who” introduces information about a person: “The teacher who helped me is retiring.” “Which” introduces information about a thing, animal, or idea: “The course which I’m taking starts in March.” This is the rule most learners already know — but it’s only the starting point. The real decisions come next.

2. Work out whether the clause is defining or non-defining — everything else depends on it

Before you pick a pronoun, ask: does this clause tell me which one, or does it just add extra detail?

• Defining (restrictive): “The teacher who gave us homework is strict.” Without the clause, you wouldn’t know which teacher.
• Non-defining (non-restrictive): “My teacher, who has taught here for 20 years, is retiring.” You already know which teacher (it’s “my teacher”); the clause just adds a fact.

This single distinction decides whether you use commas, whether “that” is allowed, and whether you can drop the pronoun. Get it right first, and the rest follows.

3. In defining clauses, who, which, and that are all fair game

For defining clauses, “that” can usually replace “who” or “which”: “The teacher that gave us homework is strict.” “The course that I’m taking starts in March.” This is why natural spoken English leans so heavily on “that” — it works for both people and things, no decisions required.

4. In non-defining clauses, never use “that” — and never skip the commas

This is the rule most learners miss, because Spanish doesn’t have an equivalent distinction. “My teacher, that has taught here for 20 years…” is wrong. It has to be who or which, and it needs commas on both sides (or one, if the clause ends the sentence): “My teacher, who has taught here for 20 years, is retiring.” Drop the commas from a non-defining clause and you change the meaning: “Teachers who work hard should earn more” (only the hardworking ones) versus “Teachers, who work hard, should earn more” (all teachers, and by the way, they work hard).