(一)情态动词无人称和数的变化，其后可跟不定式的一般式，也可跟不定式的完成式。can用在否定句中，表示否定的判断或推测以及理论上的可能性，其肯定形式为must+原形;could+原形动词表示现在或将来的推测或判断。

That can’t be true.

It must be done.

You could be right.

(二)can和could可用于下列结构：

can’t help + v-ing

cannot help but不得不

can but只得

cannot choose but只得

cannot…too (over)无论……也不过分

One cannot be too careful.

I can but say it again.

(三)can或could与不定式的完成式连用，表示说话人对过去发生的行为的真实性所持的态度(惊奇或怀疑)。can只用于疑问句或否定句。can’t/ couldn’t have + 过去分词表示对过去情况的强烈否定推测，一般译为“一定没……”。

Where can she have gone?

She can’t have gone to school. — It’s Sunday.

He could have gone off with some friends.

(四)may和might均可表示可能性和许可。may可表示给予许可，might则不能。may用于现在和将来时，might用于过去时或表示婉转意义，或可能性不大。

She may not be at home. (= It is possible that she is not at home.)

She can’t be at home. (= it is not possible that she is at home.)

Brown might come to sign up for the match 2 days later.

(五)may +不定式的完成式表示说话人对过去发生的事情的怀疑与推测;might+不定式的完成式表示对过去发生的事情可能性的推测，所推测的可能性有时与实际情况相反，一般含批评、责备的口吻。可能性最小。

She is late. She may have got up late.

You might have done it better.

(六)must表示义务时，其否定式形式为don’t need to, needn’t + V或don’t have to.

You needn’t come to see me if you don’t want to.

You don’t have to talk so loud.

(七)must + 不定式的完成式表示对过去发生的事情较有把握的推测，译为“一定……”，否定式为can’t have done.

The road is wet. It must have rained last night.

He can’t have finished all the exercises within an hour.

(八)ought to 与should + 不定式的完成式用于肯定句，表示该做而未做，用于否定句表示不该做的事而做了，有强烈的批评与责备口吻。

You ought to have handed in your homework this morning.

You should not have done such a foolish thing.

(九)needn’t have + 过去分词表示做了不必做的事，译为“其实没有必要……”。

You have five days more. You needn’t have worked so hard.