mind, do let me know. 万一你改变了想法，一定要告诉我。

Should l be free tomorrow，I will come.

= If I should be free

tomorrow, I will come.(万一我明天有时间，我就来。) Should it rain this afternoon, … 8. dare

a. 可以作情态动词，(常用于否定、疑问句和条件句)一般

不用在肯定句。有过去式，dared

How dare you say I'm unfair?He dare not speak English before such a crowd，dare he?

b. 可以作实义动词，有人称和数的变化

doesn’t dare to do c. 作实义动词并用于否定句时，to可以省略

I dare not go out alone. I don’t dare (to) go out

alone.

He dares not climb the tree. 错

He dare not climb the tree.

He doesn’t dare to climb the tree.

I dare not to swim across this river．错

He does not dare (to) answer．

Don't you dare (to)touch it? I wondered how he dared(to)

say that．

9. 情态动词+完成式

a．can／could+have done用于疑问句或否定句时，表示对过去情况的推测。Could have done也可用于 肯定句，表示\可能已经……\的意思；此外，它还可表示过去没有实现的可能性，意为\本来可以(可能)……\。 You could have done better，but you didn't try your best．