What a relative clause does
A relative clause comes straight after the noun it describes and tells you more about it. There are two kinds, and the difference is not decoration — it changes what the sentence means.
noun + relative pronoun + clause
A defining relative clause gives essential information: it identifies which person, thing, place or time you mean. Remove it and the sentence is incomplete. No commas.
The engineer who designs new software isn't here. → which engineer?
I like products that use ethically sourced materials. → which products?
A non-defining relative clause gives extra information. Remove it and the sentence is still complete and still identifies the noun. Commas always.
My phone, which cost a lot, isn't working properly.
My boss, who is very nice, is retiring next year.
Choosing the pronoun
| Refers to | Defining | Non-defining |
|---|
| a person | who, that | who (only) |
| a thing | which, that | which (only) |
| possession | whose | whose |
| a place | where | where |
| a time | when | when |
| a reason | why | — |
The key restriction: that is never used in a non-defining clause. ✗ My phone, that cost a lot, ... → ✓ My phone, which cost a lot, ...
Subject, object, and when you can leave it out
The relative pronoun can be the subject of its clause — the one doing the action — or the object — the one receiving it.
I have an uncle who is an engineer. → subject (who does the being)
I enjoyed the book that you gave me. → object (you gave it)
You can omit the pronoun in one situation only: when it is the object of a defining clause. If the clause has its own subject right after the pronoun, the pronoun can go.
I like products (that) I can use again and again. ✓ omission possible
The engineer (who) I spoke to earlier is here. ✓ omission possible
The engineer who designs new software isn't here. ✗ subject — you cannot omit it
My boss, who I have known for years, is a good friend. ✗ non-defining — you cannot omit it
The commas change the meaning
Two identical sentences say different things depending on the commas, so punctuate deliberately.
My sister who lives in Rome is a doctor. → I have more than one sister; this is the Rome one.
My sister, who lives in Rome, is a doctor. → I have one sister; Rome is extra detail.
⚠ Common mistakes:
✗ My phone, that cost a lot, broke. → ✓ My phone, which cost a lot, broke.
✗ The people which live upstairs... → ✓ The people who live upstairs...
✗ The man who he works here... → ✓ The man who works here... (no extra subject)
✗ I enjoyed the book that you gave it to me. → ✓ ...that you gave to me. (no extra object)
✗ The woman lives next door is a doctor. → ✓ The woman who lives next door... (a subject pronoun is never omitted)
✗ That's the village which I was born. → ✓ the village where I was born