定义
（语态）动词和参与此动作的主语之间关系的一个术语。当主语是动作的发起者（或之一）时，称为主动语态； 如果动词不定式的逻辑主语是这个不定式所表示的动作的承受者，不定式一般要用被动语态形式。如： It's a great honour to be invited to Mary's birthday party.（不定式作主语是被动语态to be invited是被邀请） It was impossible for lost time to be made up.（不定式作主语） I wish to be sent to work in the country.（不定式作宾语） Can you tell me which is the car to be repaired?（不定式作定语） He went to the hospital to be examined.（不定式作状语） My work is to clean the room every day.（不定式作表语）
在There be结构中，修饰主语的不定式可用被动，也可用主动。如：There are still many things to take care of (to be taken care of).但有时两种形式表达的意思不同，如：There is nothing to do now.( We have nothing to do now.) There is nothing to be done now.(We can do nothing now.)
形式
1) 现在式：一般现在时表示的动词，有时与谓语动词表示的动作同时发生，有时发生在谓语动词的动作之后。一般为：动词+ to do sth
He seems to know this.
I hope to see you again. = I hope that I'll see you again. 我希望再见到你。 2) 完成式：表示的动作发生在谓语动词表示的动作之前。 I'm sorry to have given you so much trouble. He seems to have caught a cold.
3)进行式：表示动作正在进行，与谓语动词表示的动作同时发生。 He seems to be eating something. 4)完成进行式：
She is known to have been wreaking on the problem for many years. 一般在表示情绪的动词后加to do也表将来
疑问词
疑问词who,what,which,when,where,whether,how后可接不定式构成不定式短语，在句中作主语、宾语、表 语等。如：
①When to leave for London has not been decided yet. （不定式在句子中做主语） ②Mr. Smith didn't know whether to leave or stay there. （不定式在句子中做宾语） ③I asked Professor Xu how to learn English well. （不定式在句子中做直接宾语） ④The question was where to get the medicine needed. （不定式在句子中表语）
以上例句中疑问词+不定式部分，均可转换为相应的从句形式。如：①When we shall leave…③…how I could learn……
经常在这种结构中使用的动词有：consider,decide,discover,explain,find out,forget,hear,know,lea rn,observe,understand,wonder等。
作主语
动词不定式作主语时，句子的谓语动词常用单数，其位置有以下两种： (1)把不定式置于句首。如：
To get there by bike will take us half an hour.骑自行车到那儿将花费我们半个小时。 To make up for lost time is not possible.要弥补失去的时光是不可能的。 To lean out of the window is dangerous.身子探出窗外很危险。 To save money now seems impossible.现在好像不可能存钱。
(2)用it作形式主语，把真正的主语不定式置于句后，常用于下列句式中。如： ①It+be+名词+to do
It's our duty to take good care of the old.照顾老人是我们的责任。 It is an offence to drop litter in the street.在马路上乱丢废物是违章的。 ②It takes sb+some time+to do
How long did it take you to finish the work?你花了多少时间来完成这项工作？ ③It+be+形容词+for sb+to do
It is difficult for us to finish writing the composition in a quarter of an hour.我们难以在四分之一小时内完成这篇作文。
It is stupid of you to write down everything (that) the teacher says.你把老师说的所有东西都记下来的行为是很愚蠢的。
④It seems(appears)+形容词+to do It seemed impossible to save money.
在句型③中，常用表示客观情况的形容词，如：difficult,easy,hard,important,impossible,necessary 等；在句型④中，常用careless,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise等 表示赞扬或批评的词。在不定式前的sb，可看作其逻辑主语。这一句式有时相当于Sb is+形容词+to do句式 ，如：It's kind of you to help me with my English.=You are kind to help me with my English.
⑤It + 不定式结构可位于believe/consider/discover/expect/find/think和wonder之后 He thought it would be safer to go by train.他认为乘火车比较安全。 He will find it is hard to make friends.他会感觉到交朋友困难。 ⑥不定式的完成式也可作句子的主语
To have made the same mistake twice was unforgivable.两次犯同样的错误是不可原谅的。
It is better to have loved and lost than never to have loved at all.爱过而后失去了爱，比从未爱过要好。 (3)举例
（1） It's easy (for me) to do that.我做这事太容易了
easy, difficult, hard, important, possible, impossible, comfortable, necessary, better; the first, the next, the last, the best, too much, too little, not enough It's so nice to hear your voice.听到你的声音真高兴。
It's necessary for you to lock the car when you do not use it.当你不用车的时候，锁车是有必要的。 （2） It's very kind of him to help us. 他帮助我们，他真好。
Kind, nice, stupid, rude, clever, foolish, thoughtful, thoughtless, brave, considerate(考虑周到的), silly, selfish(自私的)
例句:
It was silly of us to believe him. 我们真愚蠢，竟然相信了他。
It seemed selfish of him not to give them anything. 他不给他们任何东西，这显得太自私了。 注意：（1） 其他系动词如，look，appear等也可用于此句型 （2） 不定式作为句子成分时，动词用单数形式。
（3） 当不定式作主语的句子中又有一个不定式作表语时，不能用It is… to…的句型 (对)To see is to believe. 眼见为实。 (错)It is to believe to see.
作宾语
1) 以下动词后，只能跟不定式作宾语
afford （付得起），agree（同意），aim(力求做到)，appear（显得），arrange（安排），ask（要求），attempt （试图），care（想要），choose（决定），claim（声称），condescend（屈尊），consent（准许），decide（决定），demand（要求），determine（决心），endeavor（竭力），expect（期待），fail（未履行），help（帮助），hesitate（犹豫），hope（希望），learn（学会），manage（设法），neglect（疏忽），offer（主动提出），plan（计划），prepare（准备），pretend（假装），proceed（接着做），promise（答应），prove（证明），refuse（拒绝），resolve（解决），seem（觉得好像），swear（发誓），tend（往往会），threaten（预示），undertake（承诺），volunteer（自愿做），vow（发誓），want（想要），wish（希望）
举例：
The driver failed to see the other car in time. 司机没能及时看见另一辆车。
I happen to know the answer to your question. 我碰巧知道你那道问题的答案。 2） 动词+疑问词+ 不定式
decide,know,consider forget,learn,remember,show,understand,see,wonder,hear,find out,explain,tell Please show us how to do that. 请演示给我们如何去做。
There are so many kinds of tape-recorders on sale that I can't make up my mind which to buy.有这么多的录音机，我都拿不定主意买哪一种。
注意：疑问词带不定式在句中作成分时，谓语动词用单数。 The question is how to put it into practice. 问题是怎样把它付诸实施。
3)当复合宾语中的宾语是不定式时，先用形式宾语it代替不定式，把不定式置于补语之后，即：主语+动词+it+补语+to do句式。如：
We think it quite important for us to learn a foreign language well. He feels it his duty to help the poor. I find it difficult to learn English well.
作补语
1) 动词+宾语+不定式(to do)
常见动词：advise，allow，believe，cause，challenge，compel，declare，encourage，forbid，force， find，hire，induce，instruct，invite，like，order，permit，persuade，remind，request，require，select，send，suppose，tell，train，urge
例句：
a. Father will not allow us to play on the street. 父亲不让我们在街上玩耍。 b. We believe him to be guilty. 我们相信他是有罪的。
2) to + be 的不定式结构，作补语的动词。
常见动词：Acknowledge, believe, consider, think, declare(声称), discover, fancy(设想), feel, find, guess, judge, imagine, know, prove, see(理解), show, suppose, take(以为), understand
We consider Tom to be one of the best students in our class. 我们认为汤姆是班上最好的学生之一。 3) to be +形容词
常见动词：Seem, appear, be said, be supposed, be believed, be thought, be known, be reported, hope, wish, desire, want, plan, expect, mean…
The book is believed to be uninteresting. 人们认为这本书没什么意思。 4) there be+不定式
常见动词：believe,expect,intend,like,love,mean,prefer,want,wish,undrstand
We didn't expect there to be so many people there.我们没料到会有那么多人在哪里。 有些动词需用as 短语做补语，如regard,think believe,take,consider. We regard Tom as our best teacher. 我们认为汤姆是我们最好的老师。 Mary took him as her father . 玛丽把他当作自己的父亲。 5)秃头不定式作补语
秃头不定式， 即不带“to”的不定式，其语法功能一般在句子中作宾语的补语（宾语补足语）。 用秃头不定式作宾语补足语的常用动词如下：
口诀：“五看、三使役“，“两听、一感”要记住，若是“宾补”变“主补”，主补“to”字不能无。动词let属例外，其宾补/主补“to”均无。说明：
五看-----see/watch/notice/observe/look at；三使役-----have/make/let；两听-----hear/listen to；一感-----feel。 例句：I often see him go to school on foot.(秃头不定式作宾语补足语) He is often seen to go to school on foot.(不定式作主语补足语，要带“to”) Let him try again.---- He is let try again.(let的主补与宾补均用秃头不定式)
Find 特殊用法
Find 后可用分词做宾补，或先加形式宾语，再加形容词，最后加带to 的动词不定式。find后也可带一个从句。此类动词还有get，have。
I found him lying on the ground. I found it important to learn.
I found that to learn English is important.