Non-defining relative clauses, also known as non-restrictive relative clauses give us more non-essential information about a noun.
They are particularly common in written English. We must use commas when writing them to separate them from the rest of the sentence.
Relative pronouns link them to the noun that they are referring to. The relative pronoun cannot be omitted. It is compulsory. Finally, we cannot replace "who", "whom" or "which" with "that".
You will also learn the grammar rules for using these pronouns as a subject or as an object.
Voici un cours d'anglais pour apprendre les subordonnées relatives, en particulier, celles qui nous donnent une information supplémentaire à propos d'un nom. Cette leçon explique également les pronoms relatifs.
This video will be useful to ESL students as well as to teachers.
I am a native English teacher and the accent is British English.