3.must与have to表示“必须”

(1)must表示“敦促”或“命令”，语气比前二者更强烈，这意味着说话人是权威的一方，对别人、对自己都适用。

You must be back by ten o'clock．

你必须10点以前回来。

I really must stop smoking．

我真的必须戒烟了。

If a person is bleeding badly，you must try to stop the bleeding．(人教大纲版高二上第63页)

如果一个人流血较多，就必须止血。

(2)表示“现在必须”我们还可以用have to;表示“将来必须”用will have to;表示“过去必须”常用had to;它的现在完成时形式是have/has had to。

There are about five million species in the oceans and we have yet to learn much about them．(人教大纲版高二下第19页)

海洋里大约有500万种生物。有关这些种群的情况，我们还有许多东西需要了解。

Anyway，I suppose I will have to let you have it．(人教大纲版高三第57页)

不管怎样，这个钱是得掏给你了。

I had to brake hard and I hit another cyclist．(人教大纲版高一下第55页)

我不得不拼命刹车，还撞到了另一个骑车者。

I have had to keep it a secret for so many years．

多年来，我不得不保守这个秘密。

提示：

表示“必须”时，must与have to意义比较接近，must表主观，have to侧重客观。

He must say it in English．(I want him to do so．)

He has to say it in English．(Because he doesn't know Chinese．)

(3)must表示“必须”时的否定式有两个。

①mustn't表示不许做某事。

You mustn't talk like that．

你不要像那样讲话。

②needn't/don't need to/don't have to/haven't got to表示“不必做某事”。

You needn't do it at once．

你不必马上做。

You don't need to do it at once．

你不必马上做。

You don't have to do it at once．

你不必马上做。

You haven't got to do it at once．

你不必马上做。

提示：

表示“过去不必”的通常形式为:

You didn't need to do it at once．=You didn't have to do it at once．

你不必马上就做。

请对比：

You needn't have done it at once．

你本来是不必做的。(但是却做了)