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.
Topic preview: Online learning saves travel time and allows review, but it can reduce immediate interaction and depends on access. Face-to-face discussion makes it easier to ask follow-up questions, though some learners need more time to prepare. I prefer a blend: a short source sheet before class and discussion in class, with printed alternatives available.