Non Defining relative clauses

Non-defining relative clauses
Non-defining relative clauses (also known as non-restrictive relative clauses) give us more information about a noun.
Non-defining relative clauses are common in written English. In non-defining relative clauses we can use the following relative pronouns: who, whom, which, whose We use “who” for people only. It acts as the subject pronoun.
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.
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. We use “whom” for people only. It acts as the object pronoun.
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”.
We use “which” for things or animals. It acts as the subject or object pronoun.
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).
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.
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.”
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.”