有读者问： 请解释一下为什么must的否定式有时是mustn't，有时是needn't，有时却是can't?
表“不可”、“不应该”、“不许可”等应用mustn't, 如：
(1)You mustn't do that.
(2)You mustn't speak like that to your mother.
(3)You mustn't cross the border.
表must的反义词应用needn't，如：
(4)—Must he come?
—No, he needn't .
(5)—Must I sign this?
—No, you needn't .
表“推测”时，肯定结构用must，否定结构用can't，如：
(6)He must have lost his way. （肯定结构）
(7)He can't have lost his way. （否定结构）
