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. Whom can be the object for a person: The tutor whom I emailed replied. Who is also normal in that object position, and the object relative pronoun may be omitted in a defining clause. After a fronted preposition in formal English, use whom: the tutor to whom I sent my question. Do not use who or that immediately after to in this formal construction.