一、Ought to表示应该。如：
1.You ought to take care of him.
你应该照顾他。
2.You ought to scrap the old bike and buy a new one.
你应该扔掉这辆旧自行车，买一辆新的。
3.You ought to have a haircut.
你该去剪发了。

二、表示推测。注意与must表示推测时的区别：
He must be at home by now. （断定他已到家）
He ought to be home by now. （不十分肯定）
This is where the oil must be. （比较直率）
This is where the oil ought to be. （比较含蓄）

三、“ought to + have + 过去分词”表示过去应做某事而实际未做。如：
You ought to have asked him (but you didn't).
这时，ought to和should可以互相换用。
注意：在美国英语中，ought to用于否定句和疑问句时，to可以省略。如：
Ought you smoke so much?
You oughtn't smoke so much.

四、ought和should的区别：
ought语气略强，should较常用；ought在美国英语中用的很少，而should却相当常用；
ought属正式用语。由于ought to 没有过去式，所以在直接引语变间接引语的过程中，就不再变化.例如：
1.She failed one of the exam papers, but we ought to make allowance for the fact that she was ill.
她有一门不及格，但我们必须考虑到她当时有病。
2.Regularity ought to be observed, as regularity is very conducive to health.
生活应遵守规律，因为规律对健康有益。
3.China ought to make a greater contribution to humanity.
中国应当对于人类有较大的贡献。
4.He is a very good tennis player; you ought to see him in action.
他是个很好的网球运动员，你应当去看看他打球。
5.I think there's a train at 8.20 but you ought to make certain.
我想8点20分有一班火车，不过你应该打听清楚。
6.I think you ought to go to/to go and see the doctor.
我认为你应当去找医生看看病。