Who, Which, That: Relative Clauses Explained Simply
"The man who I saw." "The book which I read." "The car that broke down." Relative clauses let you combine ideas into one sentence — but the rules for who, which, and that confuse even advanced learners. This guide breaks it all down.
What Is a Relative Clause? (Simple Explanation)
A relative clause is a part of a sentence that gives more information about a noun. It starts with a relative pronoun — who, which, that, whose, where, or when — and acts like an adjective, describing the noun that comes before it.
Without relative clauses, you'd need two separate sentences for every detail. They're the glue that holds complex English together, and native speakers use them constantly without even thinking about it.
Two separate sentences
I met a woman. She speaks five languages.
Combined with a relative clause
I met a woman who speaks five languages.
Two separate sentences
She bought a laptop. It was very expensive.
Combined with a relative clause
She bought a laptop which/that was very expensive.
The relative clause always comes right after the noun it describes. 'I met a woman who speaks five languages' — 'who speaks five languages' describes 'woman,' not 'I.'
Who, Which, That: The Basic Rules
The choice between who, which, and that depends on what the noun is — a person, a thing, or either. Here's the core rule that covers 90% of situations.
WHO — for people
The teacher who taught me English was from London.
Use 'who' when the noun is a person (or people).
WHICH — for things and animals
The book which I borrowed was fascinating.
Use 'which' when the noun is a thing, an animal, or an idea.
THAT — for people or things
The teacher that taught me was great. / The book that I read was long.
'That' is the most flexible — it works for both people and things in defining clauses.
In everyday speech and informal writing, 'that' is far more common than 'which' for things. 'The movie that I watched' sounds more natural than 'The movie which I watched' in American English.
Defining Clauses: Essential Information (No Commas)
A defining relative clause (also called a restrictive clause) gives essential information. Without it, the sentence doesn't make sense or means something completely different. Defining clauses have NO commas.
The key test: if you remove the clause, does the sentence still identify the noun? If not, it's a defining clause.
The students who passed the exam were happy.
Which students? The ones who passed. Without the clause, we don't know which students.
The restaurant that we went to last night was excellent.
Which restaurant? The one we went to last night. The clause identifies it.
People who exercise regularly live longer.
Not all people — specifically people who exercise. The clause limits the meaning.
Rules for Defining Clauses
NO commas around the clause
Can use who, which, or that
'That' is preferred in American English for things
The relative pronoun can often be dropped (see Section 5)
Non-Defining Clauses: Extra Information (With Commas)
A non-defining relative clause (also called a non-restrictive clause) adds extra information that is NOT essential. The noun is already identified — the clause just adds bonus detail. Non-defining clauses ALWAYS have commas.
The key test: if you remove the clause, does the sentence still make complete sense and identify the noun? If yes, it's non-defining.
My sister, who lives in Paris, is a doctor.
You already know which sister (I only have one). 'Who lives in Paris' is extra info.
The Eiffel Tower, which was built in 1889, attracts millions of tourists.
There's only one Eiffel Tower. 'Which was built in 1889' is bonus information.
My car, which I bought last year, has already broken down.
You know which car (my car). The clause adds when I bought it.
Rules for Non-Defining Clauses
ALWAYS use commas (before and after the clause)
Can use who or which — NEVER 'that'
The relative pronoun can NEVER be dropped
Common with proper nouns and unique things
NEVER use 'that' in non-defining clauses. 'My mother, that is 60, loves gardening' is WRONG. Say: 'My mother, who is 60, loves gardening.'
When You Can Drop the Relative Pronoun
In defining clauses, you can often drop the relative pronoun entirely. This makes sentences shorter and more natural — and native speakers do it all the time. But you can only drop it in specific situations.
You CAN drop it when the pronoun is the OBJECT
The movie that I watched was boring.
The movie I watched was boring.
'I' is the subject, 'that' is the object → droppable
The person who you met is my boss.
The person you met is my boss.
'you' is the subject, 'who' is the object → droppable
The cake which she made was delicious.
The cake she made was delicious.
'she' is the subject, 'which' is the object → droppable
You CANNOT drop it when the pronoun is the SUBJECT
The man who lives next door is friendly.
The man lives next door is friendly.
'who' is the subject of 'lives' → cannot be dropped
The bus that goes to the airport is blue.
The bus goes to the airport is blue.
'that' is the subject of 'goes' → cannot be dropped
Quick test: look at what comes AFTER the relative pronoun. If it's a verb, the pronoun is the subject → keep it. If it's a noun/pronoun (I, you, she, they...), the pronoun is the object → you can drop it.
Where, When, Whose: The Other Relative Words
Besides who, which, and that, English has three more relative words that serve specific purposes. They replace 'in which,' 'at which,' or 'of which' and make sentences sound much more natural.
WHERE — for places
The restaurant where we had dinner was lovely.
I visited the town where I grew up.
'Where' replaces 'in which' or 'at which': The restaurant in which we had dinner → The restaurant where we had dinner.
WHEN — for times
I remember the day when we first met.
Summer is the season when most people travel.
'When' replaces 'in which' or 'at which' for time expressions: The day on which we met → The day when we met.
WHOSE — for possession
The woman whose car was stolen called the police.
I have a friend whose brother is a pilot.
'Whose' replaces possessives (his, her, its, their). It works for people AND things: The house whose roof was damaged → The house the roof of which was damaged.
In informal speech, 'when' and 'where' can sometimes be replaced with 'that' or dropped: 'The day (that) we met.' 'The place (that/where) I work.' But 'whose' can never be replaced or dropped.
"Which" Referring to a Whole Sentence
One special use of 'which' confuses many learners: it can refer to an entire previous clause or idea, not just a single noun. This is always a non-defining clause with a comma.
When 'which' refers to a whole sentence, it comments on the entire situation — not one specific noun. This is very common in both spoken and written English.
He passed all his exams, which surprised everyone.
'Which' refers to the whole fact that he passed — not to 'exams.'
She arrived late, which annoyed her boss.
'Which' refers to the fact that she arrived late.
The company raised prices, which led to fewer customers.
'Which' refers to the entire action of raising prices.
He didn't apologize, which made things worse.
'Which' refers to the whole situation of not apologizing.
You can ONLY use 'which' here — never 'that.' 'He passed his exams, that surprised everyone' is WRONG. Also, you must always use a comma before 'which' in this construction.
That vs Which: The American vs British Debate
One of the most argued-about points in English grammar is when to use 'that' versus 'which.' The answer depends on whether you're following American or British conventions.
American English (Strict Rule)
Defining clauses: use 'that' (preferred) — The car that I bought...
Non-defining clauses: use 'which' (required) — My car, which I bought last year,...
'That' and 'which' are NOT interchangeable
Most American style guides (AP, Chicago) enforce this distinction
British English (Flexible)
Defining clauses: 'that' or 'which' — both acceptable — The car that/which I bought...
Non-defining clauses: 'which' only — My car, which I bought last year,...
'Which' is used more freely in defining clauses
Academic and formal writing often uses 'which' in defining clauses
The one rule that EVERYONE agrees on: non-defining clauses (with commas) ALWAYS use 'which,' never 'that.' The debate is only about defining clauses.
For learners: follow the American rule. Use 'that' for defining clauses and 'which' for non-defining clauses. It's clearer, it's never wrong, and it helps you see the difference between the two clause types.
Common Mistakes Learners Make with Relative Clauses
These are the errors that appear most often in learner English. Most of them come from translating directly from your native language or confusing defining and non-defining clauses.
Using 'that' in non-defining clauses
My mother, that is a teacher, lives in London.
My mother, who is a teacher, lives in London.
Non-defining clauses (with commas) never use 'that.'
Adding a pronoun after the relative pronoun
The book that I read it was interesting.
The book that I read was interesting.
'That' already replaces 'it.' You don't need both.
Using 'who' for things
The car who broke down was old.
The car that/which broke down was old.
'Who' is only for people. Use 'that' or 'which' for things.
Missing commas in non-defining clauses
London which is the capital of England is very expensive.
London, which is the capital of England, is very expensive.
Non-defining clauses MUST have commas. Without them, the meaning changes or the sentence is incorrect.
Dropping the pronoun when it's the subject
The woman lives next door is friendly.
The woman who lives next door is friendly.
You can only drop the pronoun when it's the object, not the subject.
Using 'what' instead of 'that/which'
The thing what I like most is the music.
The thing that I like most is the music.
'What' is never a relative pronoun in standard English. Use 'that' or 'which.'
Building Complex Sentences: Step-by-Step Practice
Now let's practice combining simple sentences into complex ones using relative clauses. Follow the three-step method for each pair.
The 3-Step Method
Step 1: Identify the shared noun in both sentences
Step 2: Replace the noun in the second sentence with who/which/that
Step 3: Place the relative clause right after the shared noun
I have a friend. She is a pilot.
I have a friend who is a pilot.
Shared noun: friend/she → 'who' (person, subject)
We visited a museum. It was closed on Mondays.
We visited a museum that/which was closed on Mondays.
Shared noun: museum/it → 'that' or 'which' (thing, subject)
The man is my uncle. You saw him yesterday.
The man (who/that) you saw yesterday is my uncle.
Shared noun: man/him → 'who/that' (person, object — can be dropped)
She bought a dress. The dress was on sale.
She bought a dress that/which was on sale.
Shared noun: dress → 'that' or 'which' (thing, subject)
I work in a building. The building was built in 1920.
I work in a building that/which was built in 1920.
Shared noun: building → 'that' or 'which' (thing, subject)
Challenge: Multiple Relative Clauses
The teacher who taught me English, which was my favorite subject, retired last year.
You can use multiple relative clauses in one sentence — but don't overdo it. Two is usually the maximum before a sentence becomes hard to follow.
How to Master Relative Clauses With FlexiLingo
Relative clauses appear in virtually every English sentence longer than five words. The best way to internalize them is through massive exposure to real English — hearing and reading them in context thousands of times until the patterns become automatic.
Interactive subtitles on 23+ platforms
Watch YouTube, Netflix, TED Talks, and more with interactive subtitles. Notice how speakers naturally use 'the person who,' 'the thing that,' and 'which means that' in real conversations and presentations.
Save complex sentences with one click
When you hear a great example of a relative clause — like 'The company that hired me, which is based in London, has offices worldwide' — save the full sentence with audio context for review.
AI-powered grammar analysis
FlexiLingo's NLP engine identifies clause structures, relative pronouns, and sentence complexity in the content you watch. See grammar patterns broken down in real time.
Spaced repetition review
Review saved examples at optimal intervals. Hearing correct relative clause patterns repeatedly — 'who lives,' 'that I saw,' 'which was built' — builds instinctive accuracy.
Frequently Asked Questions
What is the difference between who, which, and that?
'Who' is for people: 'The woman who called.' 'Which' is for things: 'The car which broke down.' 'That' works for both people and things in defining clauses: 'The woman that called' / 'The car that broke down.' In non-defining clauses (with commas), only 'who' (people) and 'which' (things) are allowed — never 'that.'
When should I use commas with relative clauses?
Use commas when the clause adds EXTRA information that could be removed without changing the core meaning (non-defining). Don't use commas when the clause is ESSENTIAL to identify the noun (defining). Compare: 'My brother, who is a doctor, lives in Paris' (extra info, one brother) vs 'My brother who is a doctor lives in Paris' (identifying which brother — I have several).
Can I always replace 'who' and 'which' with 'that'?
Only in defining clauses (no commas). 'The man that I met' and 'The book that I read' are both fine. But in non-defining clauses (with commas), you MUST use 'who' or 'which': 'My mother, who is 60, lives in London' — NOT 'My mother, that is 60, lives in London.'
When can I drop the relative pronoun?
You can drop it only in defining clauses when the pronoun is the OBJECT (not the subject). Test: look at what follows the pronoun. If it's another subject (I, you, she, they...), you can drop it. 'The movie (that) I watched' — drop OK. 'The man who lives here' — cannot drop (who = subject of 'lives').
Is it 'the reason why' or 'the reason that'?
Both are correct and common. 'The reason why I left' and 'The reason that I left' are both natural. You can even drop both: 'The reason I left.' In formal writing, some style guides prefer 'the reason that' or just 'the reason,' but 'the reason why' is perfectly standard in all contexts.