***** NOT A TEACHER *****
Good morning, Xianyu.
(1) Like you, I am eagerly awaiting an answer from one of the great teachers.
(2) As we wait, may I share some views?
(3) As a native speaker, I get the the sense that there IS a difference,
but I cannot put my finger on it.
(4) I checked my books, and this is what I found:
(a) Of course, Isn't he a student? = informal.
(b) Of course, Is he not a student? = formal.
(c) The formal one is considered neutral in meaning.
(d) the informal one sometimes implies a negative tone.
(i) Why didn't you do the work? (You should have!) (Why did you not do the work? = does not have this negative sense)
(a) This is a example from Mesdames Celce-Murcia and Larsen-Freeman.
(5) Let's look at your sentences "Isn't he a student?" and "Is he not a student?" (The following is only my opinion)
(a) Maybe in many situations, either is correct.
(b) Maybe (as the two ladies above pointed out) the contracted one could be a bit negative:
Mrs. Smith: How is your son doing at the university ?
Mr. Jones: OK. But he's a little upset.
Mrs. Smith: About what?
Mr. Jones: About all the required reading.
Mrs. Smith: Isn't he a student? = Aren't students supposed to
read a lot? Why is he complaining?
Have a nice day!
Student or Learner