When should you use "should" and when should you use "shouldn't"?

Well, it sounded OK to me but I guess folks could argue that it was redundant.

It’s a bit more complex.

Take these two questions:

1) Shouldn’t you tell your sister that you’re sorry for hitting her?
and
2) Should you tell your sister that you’re sorry for hitting her?

They are both grammatically correct and legitimate sentences/questions – but the tone of the first one “shouldn’t” is much different from the second. The first one is guiding or prejudging the answer, and almost requires a YES answer.

The second one is more of a wide open question, to which ANY answer is legit.

I personally think that if a person start’s a question with a negative (Shouldn’t? Can’t? Won’t?”) they have prejudged the situation and anything I respond with will be tainted.

“Should” suggests that this is something that you do.
“Shouldn’t” is short for “should not” meaning you don’t.

@elbanditoroso is correct. Both are grammatically correct, but “shouldn’t” often expects a positive answer.

“Shouldn’t we go see him?” implies “I think we should go see him, how about you?”

conversely

“We shouldn’t go see him, should we?” expects a negative answer.

@elbanditoroso Interesting that you would take the negative as criticism. If I hear “shouldn’t” I’m more apt to regard it as “explain why you believe my preconceived notion incorrect? The one word is a very handy shortcut for “Am I wrong in thinking x. Explain why”

Should should be used positively and in questions.
Should shouldn’t be used negatively.
Shouldn’t should be used negatively and in questions.
Shouldn’t shouldn’t be used positively.
Should shouldn’t be where should should be.
Shouldn’t should be where should shouldn’t be.

Suppose you ask, “Should we not go?”, and the reply is “yes”, does that mean that we should go or that we shouldn’t go?