Defining relative clause
A defining relative clause (also called restrictive 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 defining relative clause helps to identify the noun that is being referred to. The following examples show the relative clauses in bold.

 

Examples:

  • That is the house which is unoccupied for years.
  • He was the man that killed a crocodile.
  • The woman who is standing over there is our tour guide.

 

The relative pronouns used above (which, that, who) act as subjects. Each of them introduces a relative clause that defines or identifies the noun/subject which precedes it. If a relative 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. 

Examples:

  • That is the house. (What is it about the house?)
  • He was the man. (What about the man?)
  • The woman is our tour guide. (Which woman is our tour guide?)

 

In defining relative clauses, that can be used to replace who, whom or which.

Examples:

  • It was a young couple who moved in next door.
  • It was a young couple that moved in next door.
  • It was her parents whom he was most anxious to meet.
  • It was her parents that he was most anxious to meet..
  • The coconut which dropped on his head nearly killed him.
  • The coconut that dropped on his head nearly killed him..

 

A relative pronoun can define the subject of the verb in a defining relative clause. When the relative pronoun is the subject, it cannot be omitted from the sentence. Using the defining relative clauses above as examples, the following show the omission of the relative pronouns.

Examples:

  • Incorrect: I think I know that woman is standing over there.
    Correct: I think I know that woman who is standing over there.
  • Incorrect: That is the house is unoccupied for years.
    Correct: That is the house which is unoccupied for years.
  • Incorrect: He was the man killed a crocodile.
    Correct: He was the man that killed a crocodile.

 

A relative pronoun can define an object of the verb in a defining relative clause. The relative pronoun which is an object can be omitted.

Examples:

  • The repairman whom you called has arrived.
  • The repairman you called has arrived.
  • The fish that you caught will be cooked for dinner.
  • The fish you caught will be cooked for dinner.

 

When, where, why and whose can be used in defining relative clauses.

Examples:

  • I regret losing those opportunities when I could have got married.
  • That is the bullring where we used to watch bullfights.
  • There are a few reasons why I can’t lend you the money.
  • That is my fat auntie whose husband is a lot fatter than her.

 

Non-defining relative clause
A non-defining relative clause (also called nonrestrictive clause) adds extra information about a noun which already has a clear reference. The information is not necessary so that if a non-defining relative clause is removed from the sentence, the sentence will still make sense. The relative pronoun that is not used in a non-defining relative clause; instead, who, whom, whose or which are used to introduce the non-defining relative clause.
 

A comma is used before a non-defining relative clause. If the non-defining relative clause is in the middle of a sentence, it is enclosed by commas.

Examples:

  • Nick, who is my good friend, has made it to the final of the competition.
  • My cousin Dick, whose father is my father’s elder brother, and I have been good mates ever since we were at school together. 

 

That is not used for a non-defining relative clause. 

Examples:

  • No: My grandfather is Mr Beardson that has a long beard.
  • Yes: My grandfather is Mr Beardson, who has a long beard.
  • No: He collects antique furniture that he started five years ago.
  • Yes: He collects antique furniture, which he started five years ago.
  • No: The coconut that I bought this morning is used for baking.
  • Yes: The coconut, which I bought this morning, is used for baking.

 

The following examples show that the relative pronouns who and which are used as the subjects of non-defining relative clauses, and that the sentences still make sense with the removal of the relative clauses.

Examples:

  • The Mona Lisa was painted by Leonardo da Vinci, who was also a prolific engineer and inventor.
  • The Mona Lisa was painted by Leonardo da Vinci.
    (The relative clause who was also a prolific engineer and inventor is a non-defining clause that is added to the main clause just give more information. Its removal does not affect the grammar of the sentence.)
  • Sam, who died last year, was our favourite uncle.
  • Sam was our favourite uncle. (The past tense verb was shows Sam is no longer living.)
  • She often walks to the library, which is not far from her house.
  • She often walks to the library.

 

The relative pronouns who, whom and which may be used as objects in non-defining relative clauses.

Examples:

  • One of our classmates is Alice, who we like.
    (Who is usually used as the subject, but sometimes it is used as an object in place of whom.)
  • His girlfriend ran away with his best friend, whom he trusted.
  • He is selling his wheelbarrow, which is a big one.

 

A non-defining relative clause may be used to describe the whole main clause.

Examples:

  • He left to work overseas, which was a great loss to us.
  • We have not had a car for a few years, which means that we have saved a lot on fuel and maintenance.

 

Difference between defining and non-defining clauses

Examples:

  • Bobby's sister, who works as a nurse, is my sister's friend.
    (Bobby has only one sister who is a nurse.)
  • Bobby's sister who works as a nurse is my sister's friend. 
    (Bobby has more than one sister, and the one who is my sister's friend is a nurse.)