English grammar: relative clauses

What is a relative clause? Here are some examples and explanations.

Relative clauses
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.* ~~Mr Fry, that* speaks five languages, works as a translator for the EU.~~* The area, which* has very high unemployment, is in the north of the country.* ~~The area, that* has very high unemployment, is in the north of the country.~~*

Relative clauses - common mistakes
Common mistakes	Correct version	Why?
~~The man who was sitting next to me he had a brown jacket.~~	The man who was sitting next to me had a brown jacket.	After a relative clause we do not repeat the subject.
~~The book what I bought was by Edward Lear.~~	The book that I bought was by Edward Lear.	Only that or which are possible, not what.
~~He lent me the book, which I found it very useful.~~	He lent me the book, which I found very useful.	A relative clause can have only one direct object.
~~The winner, that was 25, will receive $12,000.~~	The winner, who was 25, will receive $12,000.	That cannot be used in a non-defining (extra information) clause.
~~Our office is about two kilometres from the centre, which I share with my two colleagues.~~	Our office, which I share with my two colleagues, is about two kilometres from the city centre.	A relative clause follows the noun to which it refers.

Relative clauses