Restrictive and non-restrictive clauses are two types of adjective clauses. An adjective clause is a clause that acts as an adjective; that is, it modifies a noun or pronoun in a sentence.

Example: I saw a kid who was crying.

In this sentence, the clause “who was crying” is an adjective clause because it gives more information about the noun “kid.” Since restrictive and non-restrictive clauses are types of adjective clauses, they also primarily modify nouns by adding information.

A restrictive clause modifies a noun by giving additional information about it as well as by specifying it. It specifies which person or thing we are talking about.
On the other hand, a nonrestrictive clause modifies a noun only by giving additional information about it but does not specify it.

Read the following examples for a better understanding:

My friend who has red hairs won the competition.       (Restrictive Clause)
My friend, who has red hair, won the completion.         (Nonrestrictive Clause)

In the first sentence, the clause ‘who has red hair’ is written without commas. It is a restrictive clause. It gives information (having red hair) about the friend who won the competition. It also specifies the friend. It means that I have only one friend who has red hair, and that friend won the competition.

In the second sentence, the clause ‘who has red hair’ is written with commas. It is a nonrestrictive clause. It gives information (having red hair) about the friend, but it does not specify the friend. It means that I have many friends, and it can be any one of them who has red hair and won the competition.

The difference between restrictive and nonrestrictive clauses lies in the interpretation of a sentence or the intention of the writer to specify or not specify the noun. If a clause is written without commas, the writer intends to specify the noun (restrictive clause). If a clause is written with commas, the writer does not intend to specify the noun

Example 1.

• The man who is wearing a white shirt is my father. ✔ Correct
  
• The man, who is wearing a white shirt, is my father. (Changes the meaning)

In the above sentence, the clause refers to a specific person, “father.” Therefore, a restrictive clause should be used. There may be many men wearing white shirts, but the writer is referring to only one particular person—his father. For this reason, the clause is written without commas so that it clearly identifies the noun and functions as a restrictive clause.

Example 2.

• The book that is on the top shelf belongs to me. ✔ Correct
  
• The book, which is on the top shelf, belongs to me. (Changes the meaning)

In the first sentence, the clause “that is on the top shelf” is restrictive. It identifies which specific book belongs to the speaker. There may be many books, but only the one on the top shelf is being referred to.

In the second sentence, the clause is nonrestrictive. It adds extra information about the book but does not specify which book. It implies the listener already knows which book is meant and thus there is no need to specify it.

 

• ‘That’ is used to start a restrictive clause.
• ‘Which’ is used to start a nonrestrictive clause.


Examples:
The table that is made of wood is beautiful.  (Restrictive Clause)
A table, which is made of wood, is beautiful. (Nonrestrictive Clause)

This means the usage of the word ‘that’ or ‘which’ with a clause would turn it into either a restrictive or a nonrestrictive clause. In other words, a clause starting with the word ‘that’ or ‘which’ would be interpreted differently. For instance, a clause starting with the word ‘that’ not only gives more information about the noun but also specifies it. However, a clause starting with the word ‘which’ gives more information about the noun but does not necessarily specify the noun.