“My lawyer, whom I met yesterday, teaches civil law at Milan University”. Right. I know I can’t use “that” instead of “whom” in that sentence but I haven’t understood why. Can you help me? I’d be very happy of that
“that” does not work well in non-defining clauses (as you have here).
In defining clauses, some people are happy with “that” referring to people, so would be happy with “The lawyer that I met yesterday teaches civil law at Milan University”. Other people dislike this and think that “who”/“whom” should be used when referring to people.