1.
mustn't并不是must的否定词
must的否定词一般用needn't不需要
而mustn't是不可以,必须不的意思
2.
must强调主语的主观的"必须"
比如"i must go now for i am hungry"这里在客观上他并不一定要走,但是由于他饿了,他自己觉得必须走了,所以是must
另外must可以用做命令."you must go now!"而have to则不行
must还可以在肯定句中起推断的作用
比如"he must be there,is he?"表推断,他一定在那,对吗?而have to 同样不行
have to强调主语客观的必须
比如"i have to go now or i will be killed by my mother"这里,是由于客观上有某件事情或某个东西,所以必须要走,无论主观上想不想,都必须.
这里举个小对话,比如你(例子而已)上课想去WC,给老师说"sir!i must go out now!"老师会问你"do you really have to?明白了吧
mustn't表示,"一定不行",而don't have to 表示不需要.
比如 you mustn't run againt the traffic light你一定不能闯红灯!
you don't have to finish it in such a short time.你并不需要在那么短的时间内完成它!
这两题：
Look!The traffic light is red.I must stop my car.(主观）
It's still early.You don't have to hurry .don't have to 表示不需要.