Read about using non-defining relative clauses below. Then, when you are ready, try these relative clauses practice exercises.
Non-defining Relative Clauses
We use non-defining relative clauses to give extra information about something. In the first sentence below, the comma (,) shows us that who work very hard is extra information - if we take out this information, the listener will still understand which teachers we are talking about. In the second sentence there is no comma - who work very hard is necessary information because 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.
(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.
Defining or Non-defining?
Using a defining or non-defining relative clause (RC) can change the meaning of the sentence (as in the examples above). In addition, the rules about when to use who, which and that are different..
Defining RCs can use that instead of which/who.
The artist who/that sang "Holiday" in the 1980s 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" in the 1980s, 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 when it's the object of the verb).
Non-defining RCs always need 'who' or 'which'.
This is Andrew, who I'm going to marry.
(Non-defining RC. Who is always necessary.)
Now try these relative clauses practice exercises.