Non-defining relative clauses
In a non-defining clause, the information given is not essential. It tells us more about someone or something, but it does not help us to identify them or it (by the way, ...).
Some important aspects of a non-defining relative clause:
- They are always separated from the rest of the sentence by commas (brackets).
- In non-defining clauses, you can't use that instead of who, whom, which.
- You can't leave out the relative pronoun, even when it is the object of the verb in the relative clause.