Non-defining Relative Clauses
We use non-defining relative clauses to give extra information about something. In the first sentence below "who work very hard" is extra information - we are talking about all teachers. In the second sentence "who work very hard" is necessary - it explains which teachers we are talking about.
Teachers, who work very hard, should get paid more.
(non-defining relative clause: we think all teachers work hard)
Teachers who work very hard should get paid more.
(defining relative clause: only the hard-working teachers should get paid more, the lazy ones shouldn't)
A non-defining relative clause can also give extra information about a whole idea, not just one noun.
Last night my sister ate a steak, which was very unusual.
Here, the steak isn't unusual - my sister eating the steak is unusual - maybe she is vegetarian.
Note that non-defining relative clauses need commas (,) around them, while defining relative clauses don't. It's important to understand the difference between non-defining and defining relative clauses, because their grammar is different.
Defining RCs can use 'that' instead of 'which/who'.
The singer who/that sang "Holiday" is still making music.
(Defining RC. The information about the song helps us to know which singer we are talking about We can say who or that.)
Non-defining RCs never use 'that'.
Madonna, who sang "Holiday", is still making music.
(Non-defining RC. We know who Madonna is, so the information about the song isn't necessary. We can't say that.)
Defining RCs don't always need 'who' or 'which'.
That's the man (who) I'm going to marry.
(Defining RC. We don't need the word 'who').
Non-defining RCs always need 'who' or 'which'.
This is Andrew, who I'm going to marry.
(Non-defining RC. 'Who' is necessary.)
Non-defining RC Exercises