Non-defining relative clauses (also known as non-restrictive relative clauses) give us more information about a noun.
Non-defining relative clauses do not define or identify the noun.
Non-defining relative clauses are common in written English.
Example:
We visited Hyde Park, which is close to Buckingham Palace.
The underlined text “which is close to Buckingham Palace.” is the non-defining relative clause.
The non-defining relative clause gives us more information about Hyde Park.
It is not necessary to define or identify Hyde Park because there is only one Hyde Park in London.
We link the relative clause to the noun with a relative pronoun. In this example, the relative pronoun is “which“.
Punctuation rule
We use commas (,) to separate non-defining relative clauses from the rest of the sentence. Commas represent a pause.
Examples:
We visited Hyde Park, which is close to Buckingham Palace.
Hyde Park, which is close to Buckingham Palace, is closed today.
Relative pronouns used in non-defining relative clauses
In non-defining relative clauses we can use the following relative pronouns:
who
whom
which
whose
who
We use “who” for people only. It acts as the subject pronoun.
Example:
Mr Smith, who works with me, has invited me to a party. (subject pronoun)
In modern English, “who” is also commonly used as an object pronoun.
Example:
Mr Smith, who I like, has invited me to a party. (object pronoun)
Important: In very strict English, “who” is only used as a subject pronoun. In more formal English, only “whom” is used as an object pronoun.
whom
We use “whom” for people only. It acts as the object pronoun.
Example:
Mr Smith, whom I like, has invited me to a party. (object pronoun)
“whom” is only really used in very formal English. It is more common to use “who” as the object pronoun for people. In non-defining relative clauses, “who” and “whom” are NEVER replaced by “that”.
which
We use “which” for things or animals. It acts as the subject or object pronoun.
Examples:
He gave me a cake, which was on a white plate. (subject pronoun)
He gave me a cake, which I ate immediately. (object pronoun)
Normally we use “which” to give more information about a specific thing (noun).
Example: He gave me a cake, which was on a white plate.
But we can also use “which” to give more information about a whole clause.
Example 1:
He forgot to give me the cake, which was very annoying.
“which was very annoying” is not giving more information about the cake. It is giving us more information about the situation expressed in the whole clause “He forgot to give me the cake.”
Example 2:
Mark passed his driving test, which is fantastic.
“which is fantastic” is not giving more information about the driving test. It is giving us more information about the situation expressed in the whole clause “passed his driving test.”
Don’t forget the commas to separate the non-defining relative clause from the rest of the sentence.
In non-defining relative clauses, “which” is NEVER replaced by “that”.
Never use “that”
We never use the relative pronoun “that” in non-defining relative clauses.
Examples:
Mr Smith, who works with me, has invited me to a party. 
Mr Smith, that works with me, has invited me to a party. 
Mr Smith, whom I like, has invited me to a party.
Mr Smith, that I like, has invited me to a party.
He gave me the cake, which I ate immediately.
He gave me the cake, that I ate immediately.
We only use “that” in defining relative clauses.
whose
We use “whose” for people or things. “whose” acts as a possessive pronoun. This means it defines who owns something or who is related to someone.
Example:
Mark, whose brother works with me, wants to buy my house.
(Mark wants to buy my house.)
(Mark’s brother works with me.)
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