"who" is a pronoun. You use it as a subject. Like "Guess who's coming to dinner!"

that and which set off clauses. I'm sure it's similar as to the usage rules in German, since English is essentially a germanic language.

That sets off restrictive clauses, which are parts of the sentence that must be there for it to make sense.

Which sets off non restrictive clauses, which you can take out and the sentence will still make sense.

Are you sorry you asked? I have a BA in English and a masters in Linguistics.... : D