Unit 2 sports events grammar学案

　　　　　　　　　　　　　　　　情态动词

一.情态动词的语法特征

　1. 情态动词不能单独作谓语，须和行为动词或连系动词一起构成谓语。

2. 情态动词没有人称，数的变化，但有现在时或过去时两种形式，其过去时也不一定表示过去，往往表示婉转的或不确定的语气。

3. 情态动词除ought 和have 外，后面只能接不带to 的不定式。

　4. 情态动词没有非谓语形式，即没有不定式，分词等形式。

二.情态动词的基本用法

can /could

　1)表示能力，could主要指过去时间k

　　① Two eyes can see more than one.

　　② Could the girl read before she went to school?

2) 表惊异、怀疑等态度。主要用于否定句、疑问句或感叹句中。

① Where can (could) they have gone to?

② ②How can you be so careless?

3) 表示征求别人许可，相当于may

　　①Can/Could I have a look at your new pen?

　　②He asked whether he could take the book out of the reading-room.

注意：could也可表示请求，表委婉，主要用于疑问句，不可用于肯定句，答语应用can。

4) 表示推测，"也许，可能"，can't 表示"不可能"，绝对否定。

① Even a wise man can sometimes make mistakes.

② There is no light in the room. Can she have gone to bed?

　　③It can't be true.

　　④She can't have gone to school---it's Sunday.

5) could/ couldn't have done ◆本能够做而没能做//本不能够做而做了◆可能发生某事，实际上没有发生

① I could have come earlier, but my car broke down half-way. ◆You could have broken your leg.

②Somebody must have opened the cage-the lion couldn't have escaped on its own.◆用作猜测, 可能做了某事

③Don't worry-they could have just forgotten to phone. ◆She could have gone off with some friends.

6)cannot too...to...；can never too...to再...也不为过; 越...越好

cannot...enough再...也不够 cannot do...but do... ; cannot...but do只能做