Relative Clauses

A relative clause gives more information about someone or something referred to in a main clause. Some relative clauses are used to specify which person or thing we mean, or which type of person or thing we mean:

Example: The couple (who) live next to us have sixteen grandchildren.
Example: Andrew stopped the police car (that) was driving past.

Notice that we don't put a comma between the noun and a defining relative clause. Relative clauses begin with a relative pronoun: a wh- word (who,which,etc.) or that. However,sometimes we omit the wh- word / that and use a zero relative pronoun:

Example: We went to a restaurant (which/ that) Jane had recommended to us.

We prefer to put a relative clause immediately after or as close as possible to the noun it adds information to:

Example: The building for sale was the house (which) had a slate roof and was by the stream. (rather than The building for sale was the house by the stream which had a slate roof.)

When we use a defining relative clause, the relative pronoun can be the subject or the object of the clause. In the following sentences, the relative pronoun is the subject. Notice that the verb follows the relative pronoun:

Example: Rockall is an uninhabited island (which/ that) lies north west of mainland Scotland.
Example: We have a friend (who/ that) plays the piano.

In the following sentences, the relative pronoun is the object. Notice that there is a noun (or pronoun) between the relative pronoun and the verb in the relative clause. In this case, we can use a zero relative pronoun:

Example: He showed me the rocks (which/ that) he had bought back from Australia.
Example: That's the man (who/ that) I met at Allison's party.

We can also use ' whom' instead of ' who' as object, although ' whom' is very formal:

Example: She's an actress (whom) most people think is at the peak of her career.

We use 'that' as subject after something and anything; words such as all, little, much, and none used as nouns; and superlatives. (Which is also used as subject after something and anything, but less commonly.) We use 'that' or zero relative pronoun as object after these:

Example: These walls are all (that) remain of the city. (not... all which remain...)
Example: She's one of the kindest people (that) I know. (not... who I know.)
Example: Is there anything (that) I can do to help? (rather than... anything which I can do...)

You can't add a subject or object to the relative clause in addition to the relative pronoun:

Example: The man (who) gave me the book was the librarian.( not The man who he gave me...)

Notice also that adding a pronoun to the main clause in addition to the relative clause is unnecessary,although it is found in speech:

Example: A friend of mine ( who) is a solicitor helped me. (or, in speech: A friend of mine (who) is a solicitor - she helped me.)

I hope this article will be helped for learners of English.I give some usages of relative clauses to study easily.

Article source: http://eslarticle.com/pub/english-language-learning-ell/116143-Relative-Clauses.html