Model Verbs: 情态动词

一. 表示许可：有may, might, can, could。其中might. could则语气较委婉，但回答由他们引起的问句时，则不用might, could而用can。 1)May \ Might \ Can \ Could I keep the book for a few days? Yes, you can. 否定形式可用may not, can not或must not，不用might not。 2)Visitors may not (或can not/mustn’t) touch the button.

二、表示能力：can; be able to. be able to用于各种时态。在一般过去时中，若指“能够并且通过某种努力完成某桩事”，我们必须用be able to。

1. Cats can\ are able to climb trees very quickly.
2. A big fire broke out last night, but everyone was able to escape from the house.

三、表示“责任和义务”的情态动词有must(主观),have to(客观), mustn’t 表示“不可以、不允许”，而have to否定形式：not have to/ needn’t。

1. As there was no bus, we had to walk back home. (客观条件)
2. You mustn’t shout like that to your parents. (不可以、不允许)
3. You don’t have to\ needn’t work if you don’t feel well.

四、用于推测：must, may, might, can, could . 但疑问句中只能用can, could.

1. He looks pale. He__________ be ill.
2. —Who _____it be? It _____(not) be Mr. Smith, for he has just left for Paris. —It ________be Mr. Brown, for he has something to discuss with me.
3. He said he didn’t feel very well. He _____(not) come to the meeting this evening.

(1.must 2. can, can’t, must 3. may\might not)

▲注意: 表推测时 must, should (ought to) , may, could , might 可能性依次递减。should, ought to “应该…”较多地用于预测将来的事。can表示理论上的可能性，用于泛指.may, could, might表示事实上的可能性，用于特指具体的，可能将要发生的事。

1. He should be back by now. 他现在该回来了。
2. The teacher has explained it several times. There ought to be no problem for them.
3. A dog can be dangerous to you 狗可能给人带来危险。(有这种可能性) / This dog is very clever. It may (或might/could) help you find your key. 这条狗非常聪明，他可能会帮助你找到钥匙。指具体的事有可能发生。

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