不定式是动词的一种非限定形式，通常前面带有小品词to，有时也可不带to，它可以和助动词或情态动词构成谓语：

They will set up more evening schools next year. 他们明年要办更多的夜校。

We mustn't fall behind the others. 我们一定不要落在别人后面。

也可以作主语(a)、宾语(b)、表语(c)、定语(d)或是状语(e)：

a．To scold her would not be just. 责备她是不公平的。

b．We are planning to build a reservoir here. 我们计划在这里修一座水库。

C．One of our main tasks now is to mechanize agriculture. 我们当前的主要任务之一是实现农业机械化。

d．Do you have anything to declare? 你有什么要申报吗?

e. We have come to learn from you. 我们是来向你们学习的。

有时它还可以构成复合结构(类似汉语语法中的主谓结构)，作复合宾语等：

He wants all of us to make rapid progress. 他希望我们大家都迅速进步。

不定式在句子里虽能担任种种成分，但它毕竟是一个动词，因此具有动词的许多特点。它可以有自己的宾语(a)或状语(b)，来和它构成不定式短语：

a．Would you like to tell me how you overcame the difficulties? 你可不可以告诉我你们怎样克服困难的？

b．I prefer to go there by bike. 我宁愿骑车去。

还有下面种种形式：

主 动 形 式 被 动 形 式

一般式 to do; to be done;

完成式 to have done; to have been done;

进行式 to be doing; ________

完成进行式 to have been doing; _________

另外，不定式虽然在语法上不能有主语，但由于表示的是动作，在意思上是可以有主语的，这个主语可以称为逻辑上的主语(the Logical Subject)，以区别于语法上的主语。例如在We came to study．中，we是句子里的主语，同时也是to study的逻辑上的主语；又如在We'll send him to study abroad．中，him是句子里的宾语，而且是to study逻辑上的主语。不定式的逻辑上的主语，可以是句子中的主语、宾语等，也可以由一个用for引起的短语表示，

例如：It is difficult for a foreigner to learn Chinese. 外国人学汉语是不容易的。

2. 作主语、宾语和表语用的不定式

不定式(短语)常用来作主语：

To persevere means victory! 坚持就是胜利！

To complete that building in ten months was a great achievement. 那座楼10个月就盖成是一个了不起的成就。

To ignore this would be a mistake. 忽视这一点是错误的。

To see is to believe. 眼见为实。

To act like that is foolish. 这样做是愚蠢的。

To err is human, to forgive divine. 犯错误是人之常情，宽恕才难能可贵。

To know something about English is one thing; to know English is quite another.

懂一点英语是一回事；掌握英语完全是另一回事。

　

但在很多情况下，特别是在口语中，我们常常用代词it来代替它作主语，而把它移到句子后部去，这样使句子显得比较平稳：

It isn't right to gossip about others. (=To gossip about others isn't right.) 说闲话是不对的。

It is nice not to be dependent on them. 不依靠他们是好的。

It is not an easy thing to master a language. 学好一种语言是不容易的。

It seems a pity to refuse. 拒绝似乎是很遗憾的。

It only took (us) a year to complete the project. 完成这项工程只花了(我们)一年时间。

What harm can it do to give advice? 给人出主意有什么不好的?

How would it be to start tomorrow? 明天动身怎么样?

How much did cost (you) to send the telegram? 拍这份电报(你)花了多少钱?"

It made us very angry to hear him talk like that. 听他这样谈话我们非常气愤。

It feels good to be out here for a while. 出来在这儿待一待是挺舒服的。

It was beyond me to help them. 我无力帮助他们。

It was considered impossible to fail. 人们认为失败是不可能的。

It was decided to bring the matter up at the next meeting. 决定把这问题提到下次会上讨论。

　

如果要说明不定式表示的动作是谁做的，可以在不定式前加一个for引起的短语：

It is not hard for one to do a bit if good. 一个人做点好事并不难。

It is a great honour for us to be present at this rally. 我们来参加这个大会是很荣幸的。

It will be a mistake for us not to help them. 我们不帮助他们是错误的。

在以某些形容词(如kind, good, nice, wise, unwise, clever, silly, wrong, right, foolish, stupid, careless, considerate, rude, naughty, impolite等)作表语时，不定式前常可加一个of引起的短语，来说明不定式指的是谁的情况：

It's kind of you to think so much of us. (=You're so kind to...) 难为你这样替我们想。

(It is) Awfully good of you to come and meet us. 非常感谢你来接我们。

It's very nice of you to be so considerate. 你们想得这样周到是很好的。

It's unwise of them to turn down the proposal. 他们拒绝采纳这个建议是不明智的。
关键字:英语语法
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