[英语语法手册]情态动词--ought
ought
ought只有一种形式，后面须跟带to的动词不定式。Ought表示"有义务或必要"做某件事，还可表示"劝告"。如：.
You ought to follow the old man's advice.你应当听那位老人的话。
You oughtn't to smoke so much.你不应当抽烟太多。
You ought to go to the clinic at once. You don't look well.你脸色不好，应该马上到医务室去。
[注]ought表示"应该"，语气比should 强。
ought后加动词不定式的完成式时
指过去的动作。如用肯定形式(ought to have done)，表示某一件事该做而未做，相当于should have done。如用否定形式(ought not to have done)，则表示一件不该做的事情发生了，相当于should not have done。如：
He ought to have done the exercise more carefully.这个练习他应当作得更细心一些。
I ought to have returned these books to the library last week. 我上星期就应当把这些书还给图书馆。
You ought not to have been so rude.你不该这样粗鲁。
You ought not to have taken his skates without asking him.你不该没有得到他的许可就把他的冰鞋拿走。