Hi, I'm also an english learner and just took my TOEFL test about 15 days ago but I guess I can help you. Both "who" and "that" are used as relative pronouns that refer to a subject. "Whom" is a relative pronoun that refers to an object.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Tuongvanthe most usualYou are asking here about what is the most usual. In that case, I have to say that none of them is very usual at all. To express those thoughts, at least where I live, we say
TuongvanHe is the very man who/that has helped me learn EnglishYes, you can say it like that.