A subordinate clause that describes a noun. Defining relative clauses specify who or what is referred to.
Examples: That's the man who was rude to my mother.
This is the dress which I bought yesterday.
John's the bloke that she used to be married to.
It was the squirrel they had seen on the roof.
Non-defining relative clauses are enclosed by commas and just give additional information.
Examples: The children, who were really enjoying themselves, didn't want to leave the party.
The question of Brexit, which no one seemed able to solve, was in the news again.
Non-defining relative clauses always require a relative pronoun.
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