"Ought to" is used to give advice or make recommendations. It can also express assumption, expectation or strong probability, often with the idea that something is deserved.
He ought to quit smoking.
She ought to get a promotion.
"Ought not" (without "to") is used mostly in British English, Americans prefer "should not" or "had better not."
You ought not argue with your boss.
You shouldn't/ had better not argue with your boss.
Modal verbs are a special class of auxiliary verb that is used to express conditions, necessity, obligation, ability, and wishful desire. Example: Shall, Should, Will, Would, May, Might, Can, Could, Must