（1）ought to 表示“义务”。
What we ought to do is to give people memorable things according to our ability.
You ought to drive within the speed limit.It’s for your own good.
If the professor says so,then it ought to be true.
David ought to be in his office at this time of day.
(3)ought to + have + ed分词，表示该履行而未履行的义务。
I ought to have returned the books last week.Now they are overdue and I have to pay a fine.
You ought to have warned the boy of the danger of playing with fire.Then he wouldn’t have burnt his hands.
(4)否定式为oughtn’t to/shouldn’t,疑问式为should you/he/she等。
—You oughtn’t to eat between meals.
—I know I oughtn’t to,but I often get hungry.
You oughtn’t to smoke so much;you are inviting trouble.