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You have not to say that.（你用不着那样说。）
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You must not say that.（你决不可那样说。）
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第一句的 you have not to say=you need not say；第二句的 you must not say=you are not allowed to say.
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还有 have to 的否定有两种表现法，一则加用 do，一则不加 do，指常习的事情用 don’t have to，指特定的场合用 haven’t to，例如：
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We don’t have to work on Saturday afternoons.（通常在星期六下午我们不上班。）
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We haven’t to work this afternoon，because it’s Saturday.（因为是星期六，今天下午不上班。）
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宾语放在不定词的前后都是一样：
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I have a letter to write.或 I have to write a letter.（我有一封信要写。）
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英文疑难详解 92．must 用于过去时态
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英文助动词 must 的过去形是 had to，但常见在过去动词后的子句中仍然接用 must，而不用 had to，是不是 must 也可以当作过去动词用？
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【解答】是的，must 在某种条件之下是可以当作过去形用的。兹分别举例说明如下：
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（1）在间接说法（indirect speech）中，仍保留直接说法时的 must，以表示那种主观的决心，如：
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He said he must speak with his master.= He said，“ I must speak with my master.”（他说他一定要跟他老板谈一下。）
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She told them that they must get everything ready themselves.= She said to them，“ You must get everything ready yourselves.”（她要他们必须自己把一切准备好。）
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（2）虽无间接说法的形式，却有间接说法的意味时，也要用 must，如：
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He could not be idle；he must always be doing something.= He said to himself：“ I must always be doing something.”（他觉得他不能懒惰，他必须不停地做事。）
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They saw that they must awaken the public conscience.（他们明白他们必须唤醒民众的良心。）
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（3）在条件句中，must 可用作过去假设，而含有 would be bound to 之意：
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If he had looked，he must have seen the light of the approaching train.（= If he had looked，he would have been bound to see 或 he would be bound to have seen.）（如果他看了的话，他一定看到了那奔驰而来的火车的灯光。）
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Had it been otherwise， I must long ago have perished.（要不然的话，我一定早就死去了。）
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If he knew her real nature，he must hate her.（如果他知道她真实的性格的话，他一定会恨她的。）
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（4）在附属子句中有时也用 must：
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He knew suddenly who the woman must be.—— Jespersen（他突然明白了那女人一定是谁。）
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