How to use a relative clause?

Use relative clauses to provide extra information. This information can either define something (defining clause), or provide unnecessary, but interesting, added information (non-defining clause).
Relative clauses can be introduced by:
a relative pronoun: who (whom), which, that, whose
no relative pronoun, Ø.

where, why and when instead of a relative pronoun
You need to consider the following when deciding which relative pronoun to use:

Is the subject or object or possessive of a relative clause?
Does it refer to a person or an object?
Is the relative clause a defining or non-defining relative clause?

NOTE: Relative clauses are often used in both spoken and written English. There is a tendency to use non-defining relative clauses mostly in written, rather than in spoken, English.