动词不定式具有名词、形容词和副词的特征。因此在句中可以做主语、表语、宾语、宾语补足语、定语和状语。

一、不定式做主语：

1、不定式做主语一般表示具体的某次动作。===动名词doing 表示习惯的，经常的动作。

e.g:  To finish the building in a month is difficult.

To do such things is foolish.

To see is to believe. (对等)

注: 1).  不定式作主语时,谓语用单数

2). 当主语较长，谓语较短时，常用it做形式主语，而将不定式放到谓语的后面。

it做形式主语，不定式放在谓语动词之后常用于下列结构中：

(1)It is/was +adj.+of sb. to do…

(2) It is +adj.+for sb.+to do…

It is easy / difficult / hard / foolish / unwise / right / wrong / unnecessary

(3) it is +a +名词+ to do...

It is a pity / a pleasure / a pleasant thing / one’s duty / an honor / a shame / a crime / no easy job… to do

It takes (sb.) some time / courage / patience …to do…

It requires courage / patience / hard work… to do…

*注意:  probable 和 possible 均可作表语，但possible可以用不定式作真实主语， 而probable不能用不定式作真实主语。

It is probable for him to come to the meeting.(错）

It is possible for him to come to the meeting.

It is possible / probable that he will come to the meeting.

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