1.不定式的构成
1）不定式的一般式，不定式的一般式所表示的动作通常与主要谓语的动作同时或几乎同时发生，
或是在它之后发生。
They invited us to go there this summer.
他们邀请我们今年夏天去那儿。
He stood aside for me to pass.
他站到一边让我通过。
2) 不定式的完成式. 不定式的完成式所表示的动作在谓语所表示的动作之前发生，它在 句中可作表语、状语、宾语，有时也可作主语、定语等。如：
She seemed to have heard about this matter.
她似乎已听说过这件事。
I am sorry to have kept you waiting so long.
我很抱歉让你等了这么久。
I meant to have told you about it, but I happened to have an important thing to do.
我本来想告诉你这件事的，但我碰巧有一件重要的事要做。
It has been an honor for me to have traveled so much in your country.
对我来说，在你们国家旅行这么多地方是一件很荣幸的事情。
3) 不定式的进行时. 不定式的进行时表示正在进行的与谓语动词同时发生的动作。它在句中可以用作除谓语以外的所有成分。如：
He pretended to be listening to the teacher carefully.
他假装在认真地听老师讲课。
4) 动词不定式的否定形式是由not或never加不定式构成。
Try not to be late again next time.
尽量下次不要再迟到。
He wished us never to meet her again.
他希望我们永远不要再见到她。
5) 疑问词+动词不定式：
不定式和疑问词whether, what, which, whom, where, when, how, why等连用可以在句中起名词的作用，通常跟在tell, know, show, decide, learn, wonder, explain, advise, teach, discuss, find out等动词后面作宾语，有时也可以充当主语、表语等。
如：
On hearing the news, he didn't know whether to laugh or to cry.
听到这个消息，他不知道该哭还是该笑。
When to hold the meeting has not decided.
什么时候开会还没有决定。
The most important problem is how to get so much money.
最重要的事情是如何搞到这么多钱。
介词后一般不直接接不定式，但可以接疑问词+不定式短语作宾语。如：
Mary gave some advice on how to learn English.
玛丽提了一些如何学习英语的建议。
I have no idea of how to do it.
我不知道该怎么做。
2.不定式的语法作用.
1) 不定式作主语：
To see once is better than to hear a hundred times.
百闻不如一见。
To master a foreign language is not an easy thing.
掌握一门外语不是一件容易的事。
在很多情况下，人们通常用it作为形式上的主语，而把不定式短语移到谓语之后，使句子显得平稳一些。如：
It's good manners to wait in line.
排队等候是很有礼貌的。
It made us very angry to hear him speak to his mother like that.
听到他那样跟他妈妈说话我们很生气。
2) 不定式作表语：
The most important thing is to put theory into practice.
最重要的事情是把理论付诸实践。
The greatest happiness is to work for the happiness of all.
最大的幸福就是为大家的幸福而工作。
I feel it a great honor to be invited to speak at the meeting before so many students.
我觉得被邀请在会上面对这么多学生发言是一件很光荣的事情。
4) 不定式作补语：.
①不定式可以和名词或代词一起构成复合结构作动词的宾语，这时不定式被称为宾语补足语。如：
I would like you to help me with my English exercises.
我想请你帮我做英语练习。
I never expected the shoes to be worn out so soon.
我从来没想到鞋这么快就穿破了。注意：动词help后面接不定式作宾语或宾语补足语可以带to也可以不带to。如：Who can help me
(to) carry this heavy box?
谁能帮我拎这个重箱子？
②在make, let, have等使役动词和see, watch, notice, hear, feel等感官动词后的复合宾语中，动词不定式不带to。
I often hear her sing in the next room.
我经常听到她在隔壁唱歌。
They make the baby go to bed at 7:00 p.m. every day.
他们每天让孩子晚上7：00上床睡觉。
注意：当使役动词和感官动词变为被动语态时，不定式就成了主语补语，作主语补语的不定式必须加to。如：
Though he had often made his little sister cry, today he was made to cry by his little sister.
尽管他经常弄哭他的小妹妹，但今天他却被他的小妹妹弄哭了。
He is often heard to sing the song.
经常有人听到他唱这首歌。
③以be said, be reported, be known, seem, happen, prove, appear等构成谓语的句子中，动词不定式通常也可看作主语补语。如：
More than 20 people were reported to have been killed in the accident.
据报道，有20多个人死于事故。
I happened to be talking with him when he was hit by a car.
他被汽车撞的时候，我碰巧在和他谈话。
5)不定式作状语：
不定式作状语可以表示行为的目的、结果、原因、条件等。
1 不定式作目的状语，有时也可以用in order (not) to, so as (not) to结构。如：
In order to protect the young plants from the sun, Mother put them in the shade.
为了保护幼苗不被太阳晒坏，妈妈把它们放到了阴凉处。
He got up very early this morning so as not to be late for school again.
今天早上他起身很早以免上学再迟到。
2 不定式作结果状语，常见的结构有too…to, enough…to, so…as to, such…as to, only to…等。
The question is too difficult for me to answer.
对我来说，这个问题很难回答。
He said he was clever enough to deal with it by himself.
他说他足够聪明可以独自应付这件事。
Will you be so kind as to turn down the radio?
请你帮我把收音机调低一点好吗？
He woke up only to find himself in hospital.
他醒来发现自己在医院里。
③不定式作原因状语，通常用来修饰表示情感、心理状态、性格等的形容词。
They are surprised to learn of his death.
得知他死亡的消息，他们很惊讶。
We are proud to be young people of new China.
成为新中国的青年，我们感到很骄傲。
hard, difficult, easy, fit, comfortable等词也可以接动词不定式。如：
The water is not fit to drink.
这水不适合饮用。
The room is very comfortable to live in.
这个房间住起来很舒服。
2.不定式的复合结构.
It's expensive for people to use electricity for cooking.
人们用电做饭是很昂贵的。
It's careless of you to make such a mistake.
你犯这样的错误真是粗心。
They thought it impossible for us to find the lost child in the darkness.
他们认为我们要在黑暗中找到迷路的小孩是不可能的。
难点释疑：
1.不定式不带to的规则：
前面我们已经谈到，在一些使役动词和感官动词后的不定式作宾语补语时，不定式符号to常常省略，下面还有几种情况请大家注意：
1 在why引起的一些疑问句中，疑问词直接跟动词原形或not+动词原形。如：
Why spend so much money?
为什么花这么多钱？
Why not let her have a try?
为什么不再让她试一试？
②在had better（还是……最好）, had best（最好，顶好）, would rather（宁可，宁愿）, would rather…than（宁可……而不……）, would sooner（宁可，宁愿）, would sooner…than（宁可……而不……）, cannot but（不得不，必然）, cannot choose but（只得）, cannot help but（不得不）等结构后直接跟动词原形或not +动词原形。
如：
You'd better listen to your teacher's opinion.
你最好听一听老师的看法。
I would rather work than stay idle.
我宁愿工作而不愿闲坐。
Rather than ride on a crowded bus, he always prefers to ride a bicycle.
他宁愿挤公共汽车也不愿骑自行车。
Liu Hulan would sooner die than surrender.
刘胡兰宁死不屈。
One's world outlook cannot but come through in what one says and does.
一个人的世界观必然在他的言行中表现出来。
③在介词but, except之前如有动词do的任一形式，其后的动词不定式不用to。如：
Last evening I did nothing but repair my farm tools.
昨晚上我除了修理农具外，没有做其它的事情。
Now there was nothing he could do except admit defeat.
现在他只有认输。
如but之前没有do，其后的不定式则一般要加to。如：
I'm afraid we have no choice but to take a taxi.
恐怕我们别无选择，只好乘出租车了。
They desired nothing but to succeed.
他们只想成功。
2. for somebody to do和of somebody to do的用法区别：
句型“It is +形容词+for somebody+不定式”中的形容词通常强调不定式的行为属性，如：important, possible, impossible, necessary, difficult, hard, reasonable等。
It's difficult for us to finish the work within two hours.
我们要在两小时之内完成工作是很难的。
It's reasonable for them to run away so quickly.
他们这么快就逃跑了是很有道理的。
“It is +形容词+of somebody+不定式”中的形容词表示人物性格和特征。如：
kind, silly, good, unwise, clever, wrong, right, foolish, stupid, careless, rude, impolite, bold, thoughtful, honest, bad, sensible, naughty等。
It's kind of you to think so much of us.
难为你这么为我们着想。
这个男孩真傻，一直往篮子里倒水。