Retrieve and connect
A defining relative clause identifies which person or thing: The student who lent me a tablet is absent. A non-defining clause adds information and is separated by commas: Lan, who sits beside me, shared her notes. Use who for people, which for things, whose for possession; that can replace who/which in defining clauses but not after a comma in non-defining clauses. Do not repeat the subject: not the app which it helps me.