When should I use who and when should I use that?

Use who for people and which for things. That works for both people and things, but only in defining clauses and it sounds more informal. So the woman who called and the woman that called are both fine, while the book which I read and the book that I read are both fine. In a non-defining clause with commas, you must use who or which, never that.

How do I know when to put commas around a relative clause?

Ask whether the clause is essential. If it tells you which person or thing you mean, it is defining and takes no commas: the man who fixed my car is here. If it only adds extra detail you could delete without confusion, it is non-defining and needs commas: my brother, who lives in Leeds, is a nurse. Commas mean the information is optional.

When can I leave out who, which or that?

You can drop the pronoun when it is the object of a defining clause, that is, when another subject follows it. The book (that) I read and the woman (who) I met are both natural without the pronoun. You cannot drop it when it is the subject, so the man who called cannot become the man called.

What does whose mean and how is it used?

Whose shows possession, the relative version of his, her or their. It links a noun to the person or thing that owns it: the student whose phone rang, the house whose roof is red. Do not confuse it with who's, which is short for who is. Whose has no apostrophe and always sits in front of a noun.

Relative clauses in English: defining, non-defining and when to drop the pronoun

A relative clause adds information about a noun. Instead of writing two short sentences, I know a woman. She teaches French., you fold one into the other: I know a woman who teaches French. The clause who teaches French tells you something about the woman, and the word who stitches the two halves together. That is the whole job. The grammar that trips learners up is not the idea; it is choosing the right linking word, knowing when to add commas, and knowing when you can leave the linking word out altogether. This guide sorts that out.

The relative pronouns

A relative clause usually starts with a relative pronoun. There are six common ones, and each has a clear job.

who points to people. The doctor who saw me was kind.
which points to things and animals. The train which leaves at nine is fast.
that points to people or things, but it is more informal and only works in defining clauses (more on that below). The man that called. The bus that was late.
whose shows possession, for people and things alike. The student whose phone rang. The house whose door is blue.
where points to places. The city where I was born.
when points to times. The day when we met.

Get the pronoun right and most of the sentence falls into place. The next decision is the big one.

Defining vs non-defining clauses

This is the key distinction in the whole topic, and it turns on one question: does the clause tell you which one, or does it just add extra detail?

A defining clause identifies the noun. It answers "which one?" and you cannot delete it without losing the point of the sentence. It takes no commas.

The man who fixed my car is here.

Remove who fixed my car and you are left with the man is here, which man? The clause is doing essential work, so no commas.

A non-defining clause adds extra information. The noun is already clear, and the clause simply tells you more. You could drop it and the sentence would still make sense. It takes commas on both sides.

My brother, who lives in Leeds, is a nurse.

You already know which person my brother is. Who lives in Leeds is a bonus fact. Delete it and my brother is a nurse stands on its own. The commas signal that the information is optional.

The commas genuinely change the meaning. The passengers who were injured went to hospital (defining, no commas) means only the injured ones went. The passengers, who were injured, went to hospital (non-defining, commas) means they were all injured and they all went. Same words, different message.

One firm rule: that cannot start a non-defining clause. You can say the bus that was late (defining), but not my car, that is red, broke down. Use which or who after a comma: my car, which is red, broke down.

When you can drop the relative pronoun

Here is a small rule that makes your English sound much more natural. In a defining clause, you can leave out the relative pronoun when it is the object of the clause, that is, when another subject comes straight after it.

The book that I read was good. becomes The book I read was good.The woman who I met was friendly. becomes The woman I met was friendly.

In each case a new subject (I) follows the noun, so the pronoun is optional. If you can slot the pronoun back in and a subject follows it, you were allowed to drop it.

You cannot drop the pronoun when it is the subject of the clause, because then nothing is left to do the action.

The man who called is my boss. You cannot say the man called is my boss, that would mean something else. Here who is the subject of called, so it has to stay.

A quick test: look at the word right after the relative pronoun. If it is a verb (who called, which broke), the pronoun is the subject and must stay. If it is a new subject (that I read, who she met), the pronoun is the object and you can drop it. Non-defining clauses never allow the drop, comma clauses always keep their pronoun.

Summary table

Pronoun	Used for	Example
who	people	The nurse who helped me was calm.
which	things and animals	The phone which broke was new.
that	people or things	The keys that I lost were in my bag.
whose	possession	The boy whose bike was stolen cried.
where	places	The office where I work is in town.
when	times	The summer when we met was hot.

Common mistakes

Ranked by how often learners actually make them.

Comma errors. Putting commas around a defining clause changes the meaning or just reads as wrong. The people, who arrived late, missed the start says everyone was late; if you mean only the latecomers missed it, drop the commas: the people who arrived late missed the start. When in doubt, ask whether the clause is essential.
Using what instead of that. What is not a relative pronoun here. The car what I bought should be the car that I bought (or simply the car I bought). What means "the thing which" and cannot follow a noun.
Double subject. Learners often repeat the subject after the pronoun: the man who he lives next door. The pronoun who is already the subject, so the extra he has to go: the man who lives next door. The same slip appears with objects: the film that I saw it should be the film that I saw.
which for people, who for things. The teacher which teaches maths should be the teacher who teaches maths. People take who, things take which. That covers both if you want a single word.
whose vs who's. Who's is short for who is. The possessive is whose, no apostrophe: the woman whose bag was stolen, not the woman who's bag was stolen.

Practice

Choose the best word to complete each sentence. Answers are below.

That is the woman ___ teaches my daughter.
The restaurant ___ we ate last night was cheap.
My neighbour, ___ is a doctor, works long hours.
The student ___ phone rang looked embarrassed.
The keys ___ I lost were under the sofa.

Answers: 1. who (people, subject of the clause) 2. where (a place) 3. who (non-defining clause about a person, never that after a comma) 4. whose (possession) 5. that, or leave it out (the keys I lost): it is the object of a defining clause.