“禁止”时，就用must n ot。
Must I go tomorrow? 明天我必须去吗？
Yes, please.是的，请吧！
No , you needn’t. 不，你不必去。
2. have to 的主要用法
⑴ have to 有人称和数的变化，可用于多种时态中：一般现在时中当主语是 第三人称单数时要用has to，其余的人称用have to，一般过去时中要用had to； 一般将来时中则要用will have to.
She has to go to school by bus. 她不得不乘公共汽车上学。
You'll have to see the doctor if you get ill. 如果你生病了 ，就得去看医生。
⑵ have to 的否定句和疑问句的构成往往要借助于助动词do的适当形式或助动词will即have to, has to, had to和will have to的否定 式分别为don't have to, doesn't have to, didn't have to和won't have to，疑问句是在句首加助动词do的相应形式，句中还原成have to或把will放到句首 。
What does she have to do? 她必须做什么？
You didn't have to wait for me yesterday. 昨天你没有必要等我。
