Form and rules
How it works
1
Defining clauses identify the noun and normally have no commas; non-defining clauses add extra information and are set off by commas.
2
That can replace who/which in many defining clauses but not normally in non-defining clauses or directly after a preposition.
3
Object relative pronouns can be omitted in defining clauses; subject pronouns and pronouns in non-defining clauses cannot be omitted.
4
Formal preposition placement uses preposition + whom/which; conversational English often strands the preposition at the end.
5
Which can refer to an entire previous clause in a non-defining structure: He arrived late, which annoyed everyone.