What is the difference between who, which, and that?

Use "who" for people (The woman who called is my sister), "which" for things and animals (The book which I bought is interesting), and "that" for both people and things in defining clauses (The car that I drive is old). Note: "that" cannot be used in non-defining clauses (with commas).

When can I omit the relative pronoun?

You can omit who/which/that when it is the OBJECT of the relative clause, not the subject. Compare: "The book (which/that) I read was good" - can omit (book is object of "read"). "The man who called is here" - cannot omit (man is subject of "called"). This creates contact clauses.

How do I use whose, where, and when in relative clauses?

Use "whose" for possession (The man whose car was stolen called the police), "where" for places (The restaurant where we met is closing), and "when" for times (I remember the day when we first met). These can also be replaced: whose = of which, where = in which/at which, when = on which/in which.

What are reduced relative clauses?

Reduced relative clauses remove "who is/which was" and keep the participle for more concise writing. Active verbs use -ing: "The man (who is) waiting is my uncle." Passive verbs use -ed: "The book (which was) written by Hemingway is famous." This is common in B2-C1 level formal writing.

Why are relative clauses important for English learners?

Relative clauses are essential for: 1) Adding detail to sentences naturally, 2) Combining short sentences into complex ones, 3) Formal and academic writing, 4) IELTS, Cambridge, and other exam success (B1-C1 levels). They help you sound more fluent and sophisticated. Our 17 exercises (345 questions) progress from A2 basics to C1 advanced reductions.

Relative Clauses (Who, Which, That)

Master relative pronouns with 17 progressive exercises and 345 practice questions

Exercises

345

Questions

4-5h

Total Time

–/17

Completed

Start with Who vs Which Take Final Test

All 17 Exercises

Who vs Which

easy

Choose between who (people) and which (things)

20 questions

Start Exercise

That, Whose, Where, When

medium

Practice all relative pronouns in context

20 questions

Start Exercise

All Pronouns Mixed

medium

Choose the correct pronoun from all options

20 questions

Start Exercise

Combining: People

medium

Join sentences about people using who/that

20 questions

Start Exercise

Combining: Things

medium

Join sentences about things using which/that

20 questions

Start Exercise

Combining: Whose & Where

hard

Join sentences using whose (possession) and where (place)

20 questions

Start Exercise

Combining: Mixed

hard

Join sentences using any relative pronoun

20 questions

Start Exercise

Defining Clauses

medium

Essential information without commas

20 questions

Start Exercise

Non-Defining Clauses

hard

Extra information with commas (no 'that')

20 questions

Start Exercise

Defining vs Non-Defining

hard

Distinguish essential from extra information

20 questions

Start Exercise

Subject vs Object + Omission

hard

When can relative pronouns be omitted?

20 questions

Start Exercise

Whom (Formal Object)

hard

Formal object pronoun in writing and speech

20 questions

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Prepositions in Clauses

hard

Formal vs informal preposition placement

20 questions

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Reduced Relative Clauses

hard

Shorten clauses with participles (-ing, -ed)

20 questions

Start Exercise

Mixed Practice - Easy

easy

Review basics: who, which, that, where

20 questions

Start Exercise

Mixed Practice - Advanced

hard

Complex clauses with all features

20 questions

Start Exercise

Final Test

hard

Comprehensive test of all relative clause types

25 questions

Start Exercise

What are relative clauses in English?

Relative clauses are dependent clauses that give more information about a noun in the main sentence. They begin with relative pronouns (who, which, that, whose, whom) or relative adverbs (where, when, why). Corpus research shows relative clauses appear in approximately 15% of all English sentences, making them essential for fluent communication.

Who / Whom / Whose

For people

"The doctor who treated me was kind."

Which

For things and animals

"The book which I read was fascinating."

That

For people OR things (defining only)

"The house that we bought is beautiful."

Where / When / Why

For place, time, reason

"The city where I was born is small."

What is the difference between who and which?

The rule is simple: use "who" for people and "which" for things and animals. However, "that" can replace both in defining (restrictive) clauses. Corpus analysis shows "that" is actually more common than "who" or "which" in spoken English, especially in American English.

Who = People

"The teacher who teaches math is excellent."

Which = Things

"The book which won the award is now a film."

Which = Animals

"The cat which lives next door often visits us."

That = Both (Defining Only)

"The man that helped me was very kind." / "The car that I bought is red."

What is the difference between defining and non-defining relative clauses?

Defining (restrictive) clauses identify which person or thing we mean - they are essential to the meaning. Non-defining (non-restrictive) clauses add extra, optional information - they use commas and cannot use "that". This distinction is crucial for IELTS and academic writing.

Defining (Essential)	Non-Defining (Extra Info)
No commas	Commas required
Can use "that"	Cannot use "that"
Identifies WHICH one	Adds EXTRA information
"The student who passed got a prize."	"John, who passed, got a prize."
Essential - cannot be removed	Optional - can be removed

Key rule: If the clause identifies the noun (answers "which one?"), it's defining. If it just adds interesting information, it's non-defining. Proper nouns (names) always need non-defining clauses.

When can relative pronouns be omitted?

Relative pronouns can only be omitted when they are the OBJECT of the relative clause. When the pronoun is the SUBJECT, it cannot be omitted. This creates "contact clauses" or "zero relative clauses." Corpus research shows omission is very common in spoken English.

Object pronoun - CAN omit

"The book (which/that) I read was good."-"I" is the subject, so pronoun can go

Subject pronoun - CANNOT omit

"The book won the prize was good."-"which/that" IS the subject - must keep it

Object with people - CAN omit

"The man (who/that) she married is a doctor."-"she" is subject, pronoun can go

Test: If there's a subject AFTER the gap where the pronoun would go, you can omit it. "The book ___ I read" (subject "I" after) = can omit. "The book ___ won" (no subject after) = cannot omit.

How do you use whose in relative clauses?

"Whose" shows possession and replaces his/her/their/its. Unlike who/which, it can be used for both people AND things. It is always followed by a noun (the possessed thing), never a verb. This is a common error for learners.

People

"The man whose car was stolen reported it."

(= his car)

Things

"The house whose roof leaked was repaired."

(= its roof)

Common error: "The woman whose is my teacher..." - WRONG. Whose must be followed by a noun: "The woman whose class I attend..."

When should I use whom instead of who?

"Whom" is the object form of "who," used in formal English. It is required after prepositions: "to whom," "with whom," "for whom." In informal speech, "who" often replaces "whom," and prepositions move to the end. Use "whom" in academic writing and formal contexts.

Formal (with whom)	Informal (who...to)
"The person to whom I spoke was helpful."	"The person who I spoke to was helpful."
"The colleague with whom I work is talented."	"The colleague who I work with is talented."
"The candidate for whom I voted won."	"The candidate who I voted for won."

What are the most common relative clause mistakes?

The most frequent errors are: using "that" in non-defining clauses, omitting subject pronouns, confusing "whose" with "who's," and comma errors. Understanding these mistakes helps you avoid them.

Using "that" in non-defining clauses

"London, that is the capital, is expensive."="London, which is the capital, is expensive."

Omitting subject pronouns

"The man lives here is my uncle."="The man who lives here is my uncle."

Confusing "whose" and "who's"

"The woman who's car is red..."="The woman whose car is red..."

Missing commas in non-defining

"My sister who is 25 works here."="My sister, who is 25, works here."

Frequently Asked Questions

Sources

Research and corpus data referenced on this page:

*
Cambridge Grammar of English - Comprehensive reference for relative clause usage patterns, including defining vs non-defining distinctions.Cambridge Dictionary
*
COCA Corpus Analysis - Research on relative pronoun frequency showing "that" is more common than "who/which" in American English spoken contexts.
*
Swan (2016) - Practical English Usage - Standard reference for relative clause omission rules and contact clause patterns.
*
British National Corpus - Analysis of formal vs informal register differences in relative clause usage, particularly whom vs who.