情态动词表推测时：

1.can只能用于否定句和疑问句

2.must只能用于肯定句（它的否定句和疑问句其实就用can来代替了）

3.如句中有情态动词 + 完成时，定是对过去的推测。

4.句中如有表示不肯定的话语：I am not sure; I don't know 之类，常选may /might的各种形式。

5. "could +have done"除了表示对过去的行为进行推测外，还可表示曾有能力做某事，但并没有做，或者表示某事可能发生但并没有发生，含有轻微的责备或批评的意义，常译作"本来可以"、"本来能"等。这一用法一般只用于肯定句中。

Eg. I could have passed the exam, but I didn't study hard.我本来能够通过考试，但却没努力学习 4. 表示反劝的特殊的表推测形式

1). could + have + P.P.

表示本来能做到，但事实上没有做到。 He could have finished the task on time, but the heavy snow came. 2) . couldn't +have + P.P.

表示本来不能做到，但已经做到了。 She could not have covered the whole distance, but in fact she arrived ahead of time. 3) needn't + have + P.P.

表示本不必做的，但已经做到了。 She needn't have attended the meeting yesterday, but she did. 4. ) should/ought to +have + P.P.

表示该做而没有做 The plant is dead. I should/ought to have given ot more water. 5) shouldn't/ oughtn't to +have + P.P.

表示不该做而做了。 You oughtn't to / shouldn't have taken her bike without permission. 5. 注意：

must只能用于肯定句（它的否定句和疑问句其实就用can来代替了） 1. must + have + P.P.

表示对过去肯定的推测，"一定是，准是" The road is wet. It must have rained yesterday. 2. can + have + P.P.

表示对过去的推测（限于问句中） Can she have said so? 他可能这样说吗？ 3. can't + have + P.P.

表示对过去的否定推测 He cannot have said such a foolish thing. 二、情态动词表推测的反意疑问句

1. 情态动词表推测的反意疑问句，简单来说，就是以情态动词后的时态为淮，如句子里有明确的时间状语，则以其为准。

2. 以 must 为例：

Eg.1. You must be hungry now, aren't you?

2. He must be watching TV , isn't he ?

3 Tom must have lived her for a long time, hasn't he ?

4. She must have arrived yesterday, didn't she?

注：如选择题中（以She must have arrived yesterday, didn't she？为例）既有didn't she又有hasn't she则以didn't she？为最佳答案。

1. It______ be the vocabulary that caused you the problem in the exercise because you know a lot of words.