Explain the role of 'who' and 'whom' in non-defining relative clauses.

In non-defining relative clauses, 'who' and 'whom' play the role of introducing additional information about a person or people mentioned in the main clause. These relative pronouns are used to provide extra details or descriptions about the antecedent (the noun or pronoun that the relative clause refers to) without changing the essential meaning of the sentence.

The relative pronoun 'who' is used to refer to the subject of the relative clause. It is used when the antecedent is a person or people. For example:

- John, who is my best friend, is coming to the party.
In this sentence, 'who' introduces additional information about John, providing the fact that he is the speaker's best friend. The relative clause "who is my best friend" is non-defining because it is not essential to the meaning of the main clause, which is "John is coming to the party."

On the other hand, 'whom' is used to refer to the object of the relative clause. It is also used when the antecedent is a person or people. For example:

- I invited Sarah, whom I met at the conference, to join our team.
In this sentence, 'whom' introduces additional information about Sarah, indicating that the speaker met her at the conference. The relative clause "whom I met at the conference" is non-defining because it provides extra information but is not necessary for understanding the main clause, which is "I invited Sarah to join our team."

It is important to note that 'whom' is less commonly used in modern English, and 'who' is often used instead, even when referring to the object of the relative clause. This is because 'whom' can sound formal or old-fashioned. However, in more formal writing or when following strict grammatical rules, 'whom' is still used as the object pronoun.

In summary, 'who' and 'whom' in non-defining relative clauses serve to provide additional information about a person or people mentioned in the main clause. 'Who' is used to refer to the subject of the relative clause, while 'whom' is used to refer to the object.