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. See Unit 91 of English Grammar in Use (blue book) the explanation about Relative clauses - 'extra information' clauses and this site:http://speakspeak.com/resources/english ... ve-clausesRelative clauses
What is a relative clause? Here are some examples and explanations.
Defining relative clauses are used to specify which person or thing we mean. We don't put commas between the noun and a defining relative clause.
Who or that are used for people.
Which or that are used for things.
I have a friend who speaks five languages.
I have a friend that speaks five languages.
She showed me the coat which she had bought.
She showed me the coat that she had bought.Non-defining relative clauses (extra information clauses) are used to add extra information to a sentence. We put commas between the noun and a non-defining relative clause.Who - is used for people.
Which - is used for things.
That - cannot be used.
Mr Fry, who speaks five languages, works as a translator for the EU.
The area, which has very high unemployment, is in the north of the country.
Those firemen, who saved the little girl from the fire, are local heroes.