A relative pronoun can be used to introduce to two types of clauses called restrictive and non-restrictive relative clauses.
Restrictive / defining relative clause
A restrictive relative clause (also called a defining relative clause) gives information about the noun which can be someone or something in the main clause. The restrictive clause usually comes after the noun, and it cannot be removed from the sentence without affecting its meaning. The word that is used much more frequently than other relative pronouns such as who, whom and which in a restrictive clause. By using that, it makes the clause essential to the meaning of the sentence. Commas are not used in restrictive clauses. Whose may also be used.
A restrictive relative clause (also called a defining relative clause) is essential to the meaning of the sentence. It gives information about the noun which can be someone or something in the main clause. It usually comes after the noun. The defining relative clause cannot be removed from the sentence without affecting its meaning. The word that is used much more frequently than who, whom or which in a defining relative clause. Whose may also be used in both types of clauses. Commas are not used in defining relative clauses.
The information given by a restrictive relative clause helps to identify the noun that is being referred to. The following examples show the restrictive clauses in bold.
The relative pronouns used above (which, that, who) act as subjects. Each of them introduces a restrictive clause that defines or identifies the noun/subject which precedes it. If a restrictive clause is removed, the noun (house, man, woman) is no longer identified and as a result, the sentence does not make sense or in some cases, it gives a different meaning.
In restrictive clauses, that can be used to replace who, whom, or which.
A relative pronoun can define the subject of the verb in a restrictive clause. When the relative pronoun is the subject, it cannot be omitted from the sentence. Using the restrictive clauses, the following show the omission of the relative pronouns.
A relative pronoun can define an object of the verb in a restrictive clause. The relative pronoun which is an object can be omitted.
When, where, whose and why can be used in restrictive relative clauses.
Non-restrictive / non-defining relative clause
A non-restrictive relative clause (also called non-defining clause) adds extra information about a noun which already has a clear reference. The information is not necessary so that if a non-restrictive clause is removed from the sentence, the sentence will still make sense. The relative pronoun that is not used in a non-restrictive clause; instead, who, whom, whose or which is used to introduce the non-restrictive clause.
A comma is used before a non-restrictive clause. If the non-restrictive clause is in the middle of a sentence, it is enclosed by commas.
That is not used for a non-restrictive clause.
The following examples show that the relative pronouns who and which are used as the subjects of non-restrictive clauses, and that the sentences still make sense with the removal of the non-restrictive relative clauses.
The relative pronouns who, whom and which may be used as objects in non-restrictive clauses.
A non-restrictive clause may be used to describe the whole main clause.
Difference between restrictive/defining and non-restrictive/non-defining clauses.