Defining/non-defining clauses
- A defining relative clause tells which noun we are talking about:
I like the woman who lives next door. (If I don't say 'who lives next door', then we don't know which woman I mean).
- A non-defining relative clause gives us extra information about something. We don't need this information to understand the sentence:
I live in London, which has some fantastic parks. (Everybody knows where London is, so 'which has some fantastic parks' is extra information).