" defining relative clauses
Defining relative clauses identify the noun being referred to. They tell us which person or thing is meant. They follow immediately after the nouns that they modify with no pause in speech, so no comma is used
The boy (that) I dated when I was at school has now married my best friend.
The shares (that) he bought when he joined the company are now worthless.
Note that if we leave the identifying relative clauses out, the sentences do not have very much meaning:
The boy has now married my best friend. (Which boy?)
The shares are now worthless. (Which shares?) non-defining relative clauses
Non-defining relative clauses do not identity what is being talked about. They simply add information about the person or thing already identified. Non-defining relative clauses are normally marked
by pauses or intonation breaks and therefore also by commas
My friend Deborah, who was my closest friend in primary school, has now married my cousin.
Trevor Nunn's production of 'Anything Goes', which opened at the National only two weeks ago, has nearly sold out.
Note that if we take these non-defining relative clauses out, the sentences still make perfect sense provided the context is known:
My friend Deborah has now married my cousin.
Trevor Nunn's production of 'Anything Goes' has nearly sold out."http://www.bbc.co.uk/worldservice/learningeng...