有一读者问： 书上讲对 must 的否定应该用 needn't，但为什么下一句中用 mustn't：
(1)I must return it this week, mustn't I?
这是两个不同的问题，应分开来谈。当 must表“有必要”时，其否定结构应用 needn't，如：
(2)—Must I go?
—No, you needn't .
(3)The teacher must show concern for the pupils, needn't he?
但如 must 表“必须”时，其否定结构则须用 mustn't,如上述例（1），再如：
(4)We must go now, mustn't we?
(5)The children must wait outside, mustn't they?
又，must表推测意谓“一定”时，其否定结构一般也不用 mustn't，而须用助动词（即通常用的 do, be 或 have）的否定式，如：
(6)You must have seen him yesterday, didn't you?
(7)You must be a writer, aren't you?
(8)You must have lived here a long time, haven't you?
