Using relative clauses
Have you ever wondered about when to use that and when to use which or who in this type of sentence? In fact, for much of the time that is interchangeable with either of these words. For example:
√ You’re the only person who has ever listened to me.
√ You’re the only person that has ever listened to me.
√ It’s a film that should be seen by everyone.
√ It’s a film which should be seen by everyone
When referring to something, rather than someone, that tends to be the usual choice in everyday writing and conversation in British English. However, there is one main case when you should not use that to introduce a relative clause. This is related to the fact that there are two types of relative clause: a restrictive relative clause and a non-restrictive relative clause.