A relative clause is a clause (a group of words with a verb) that gives information about a noun (a person, thing, place, time) in the main sentence. It “relates” to that noun. Because it cannot stand alone as a sentence, a relative clause is a subordinate clause.
For example:
I saw the man who was walking his dog.
The relative clause who was walking his dog gives information about the man.
Relative clauses help us combine sentences and avoid repetition. As Cambridge Dictionary states: “Relative clauses give us more information about someone or something. We can use relative clauses to combine clauses without repeating information.” Cambridge Dictionary
Types of Relative Clauses: Defining vs Non-defining
Relative clauses come in two main types: defining (also called restrictive) and non-defining (non-restrictive). The difference is whether the clause is essential to identify the noun or simply adds extra information.
Defining Relative Clauses
A defining clause is essential: it tells us which one we mean. Without it, the meaning is unclear or ambiguous.
- No commas are used.
- You cannot omit the relative pronoun when it is the subject (in most cases).
- In some cases, you can omit the pronoun when it is the object.
Examples:
- The car that I bought is red.
- The woman who called you is my sister.
- The house which Jack built is still standing.
- The artist whom we met is famous.
Here, that I bought, who called you, etc., define which car, which woman, which house, which artist.
Non-defining Relative Clauses
A non-defining clause adds extra information about a noun that is already clear. It is not needed to identify the noun.
- Use commas to set it off from the main clause.
- You cannot omit the relative pronoun.
- You cannot use “that” in many styles (you use “who,” “which,” or “whose”).
Examples:
- My brother, who lives in Sydney, is visiting.
- The Eiffel Tower, which stands in Paris, is famous.
- Dr. Smith, whose research I admire, taught that class.
Here, the clause gives extra detail but does not define which brother or which tower—we already know.
Cambridge has a detailed account of these two kinds. Cambridge Dictionary
Relative Pronouns and Relative Adverbs
To begin a relative clause, we use relative pronouns or relative adverbs. The choice depends on what the relative clause refers to (person, thing, place, time) and its role in the clause (subject, object, possessive, etc.). See a good reference from the British Council on relative pronouns. LearnEnglish – British Council
Here are the common ones:
| Refers to / Role | Use this | When / Notes |
|---|---|---|
| Person, subject | who | The person who answered. |
| Person, object | whom (formal) or who (informal) | The person whom I saw / The person who I saw. |
| Thing / animal, subject or object | which | The car which is old. / The car which I saw. |
| Person or thing, any role | that | The book that I read / The person that helped me. (Often used in defining clauses) |
| Possessive | whose | The writer whose book I read. |
| Place | where | The town where I live. |
| Time | when | A day when everything changed. |
| Reason / cause (less common) | why | The reason why she left. |
Some rules and options:
- For a person, you can often use who or that in defining clauses.
- For things, which or that can be used in defining clauses.
- In non-defining clauses, that is rarely used; instead use who or which.
- If the relative pronoun is the object of the relative clause, you may omit it (in defining clauses) if the meaning remains clear. Example: The book (that) I bought is good.
- You cannot omit the relative pronoun in non-defining clauses.
- If the relative clause refers to a whole clause or idea (not a single noun), use which. Example: She failed the test, which surprised me. Cambridge Dictionary
- When a preposition is part of the relative clause, you may place it before the pronoun (formal) or at the end (informal / conversational).
- Formal: the person to whom I spoke
- Informal: the person whom I spoke to or the person who I spoke to
- Be careful: you do not use another object after a relative pronoun when it is already the object. For example: They went to the restaurant that Mark had been to.
(Not: … had been to it) Cambridge Dictionary
How to Use Relative Clauses (Step by Step)
Here is a step-by-step guide:
- Decide what you want to join.
Example: I saw a dog. The dog chased me. - Choose which noun the second clause will describe.
Here, “the dog” is described. - Decide whether the extra clause is essential (defining) or extra (non-defining).
If I say “the dog that chased me”, that is essential—there may be many dogs. If I say “my dog, which chased me”, that is extra info (less common here). - Select a relative pronoun or adverb.
For “dog” (thing/animal), I can use which or that. I choose that (common in defining). - Insert the relative clause immediately after the noun.
I saw a dog that chased me. - If the pronoun is the object, check whether you may omit it.
Here, “that chased me”: “that” is subject of “chased,” so we cannot omit. If it had been chased by someone, we couldn’t. But if it were the dog (that) I saw, that is object and could be omitted: the dog I saw. - Use commas if it is a non-defining clause.
More Examples
Defining
- Students who study hard usually pass.
- The house that Jack sold was expensive.
- I know the friend whose car was stolen.
- The place where we met is quiet.
- She remembers the day when we first spoke.
Non-defining
- My uncle, who is a doctor, lives abroad.
- The Taj Mahal, which was built in the 17th century, attracts many visitors.
- Shakespeare, whose works remain alive, influenced modern writing.
- Paris, where I studied, is beautiful in spring.
- April 15, when taxes are due, worries many people.
One common error: do not use a pronoun or noun inside the clause if the relative pronoun already serves that role. Wrong: That’s the school that it offers fine arts. Correct: That’s the school that offers fine arts. Cambridge Dictionary
Another error is using commas in defining clauses — avoid that. Cambridge Dictionary
Why Use Relative Clauses?
Relative clauses let us:
- Combine sentences: I met a woman. She sells flowers. → I met a woman who sells flowers.
- Avoid repetition: I lost the keys. The keys were new. → I lost the keys that were new.
- Add qualifying information: Book that I borrowed from you is good.
- Give extra details: Mary, who loves music, will join us.
They make speech and writing more fluid and precise.
What to Watch Out For
- Omitting incorrectly
You can omit a relative pronoun only when it is the object in a defining clause.
Wrong: She is the one (who) helped me. (Here “who helped me” has “who” as subject; do not omit.) - Misplacing the clause
The relative clause must follow immediately after the noun it modifies.
Wrong: The man I met yesterday, who is tall, is kind.
Better: The man, who is tall, I met yesterday.
But clearer: The man who is tall I met yesterday. - Wrong pronoun
Use who for people, which for things (in formal use). Be careful with whom and whose.
Note: In modern informal English, many speakers avoid “whom” and use “who” instead. - Comma misuse
Don’t use commas in defining clauses; use them in non-defining.
Wrong: The car, that I bought, …
Correct: The car that I bought … - Dangling prepositions (less formal)
In informal use, you may leave a preposition at the end: the person I talked to. In formal style, you may put it before the pronoun: the person to whom I talked.
Practice Tips
- Take two simple sentences and try to merge them with a relative clause.
- Decide whether the clause is defining or not, and whether to use commas.
- Choose a suitable relative pronoun or adverb.
- Check if you can omit the pronoun (only when it is the object and it is defining).
- Read examples from grammar sources like Cambridge Grammar and British Council for further models. Cambridge Dictionary
- Use worksheets and exercises — for instance, ESL worksheets on defining clauses. iSL Collective
Conclusion
Relative clauses are powerful tools in English. They let us add detail, avoid repetition, and craft clear, compact sentences. The key steps are:
- Identify the noun you wish to describe.
- Decide if the clause is essential (defining) or extra (non-defining).
- Choose the correct relative pronoun or adverb.
- Place the clause directly after the noun.
- Use commas only for non-defining clauses.
- Know when omission is allowed (only when the pronoun is an object and the clause is defining).
With practice, relative clauses become natural. If you want exercises or worksheets, I can send you some. Also, for further reading, check Cambridge’s grammar page on relative clauses.