Relative pronouns in English include that, who, which, whose, and whom. A relative pronoun introduces a relative clause, which is a subordinate clause that adds meaning to a main clause:

I borrowed a book that got exceptionally good reviews.

 

That is used to refer to people, animals, and things. It can be both singular and plural:

Customers that don’t return their books on time will get a fine.

Who refers to people (the person who wrote this essay), and its object form is whom (the girl whom I saw). Which refers to things (the answers which you gave me), Both who and which can be used in singular and plural and can be replaced by that. Which is also used when the relative clause refers to the whole main clause. Then you need to add a comma before which:

Emily is lending me her bicycle, which is nice.

Relative pronouns can be left out when the subject of the main clause is not the same as the subject of the relative clause:

The tutorial (that) I attend every Monday is cancelled.