1.不定式在句子中充当的作用
1)作主语
不定式作主语：一般表示具体的某次动作。
To complete the 30-storied building in one year was quite a difficult task.
To do that implies taking responsibility.
For there to be so modern a library in this rural area is surprising.
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 is important for modern young people to master at least two foreign languages.
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：
a) It is+形容词(easy, important, difficult, foolish, inconvenient, unnecessary, right, wrong...)+( for/of sb.)+不定式
It is essential to reserve a table in advance of Christmas Eve.
It is hard to put my hopes into words.
It is not easy to catch fish with your hands only.
It is important for us young people to learn English and master it.
b) It is+名词(a pleasure, a pity, a pleasant thing, one's duty, an honor, a shame, a crime, no easy job...)+不定式
It is a sheer waste of time to read that kind of trash.
It is a pity to have to go without her.
It is a glorious death to die for the people.
c) It takes/took (sb.) some time (hours, months, days, a lot of time, patience...)+不定式
It takes me three hours to learn English each day.
It took them half the night to get home in the snow.
2)作表语
不定式作表语，常表示将来的动作，主语常常是表示意向、打算、计划的词，如wish, idea, ask, purpose, duty, job等。
The most important thing for one's health is to have plenty of exercise.
My chief purpose has been to point out the difficulties of the matter.
The purpose of the organization is to greet all new comers to the city and to provide them with any necessary information.
What I wanted was to get the work done as quickly as possible.
注意：当主语中有do的任何形式时，作表语的不定式中的to通常省略。
What he wants to do most is (to) enjoy himself every day.
What they did last night was (to) play cards to their heart’s content.
3)作宾语
不定式作宾语，通常用在下列结构中：
a)“动词+不定式”结构：不定式直接跟在动词后面。
这类动词有：afford, agree, apply, arrange, ask, attempt, beg, begin, care, choose, claim, consent, demand, decide, desire, determine, expect, fail, hope, hesitate, hate, intend, learn, like, manage, mean, neglect, offer, plan, prepare , pretend, promise, refuse, resolve, seek, tend, threaten, want等。
I like to go out for walks in the warm sunshine in spring.
My mother hates to move from place to place, for she feels tired.
I decide to work hard and get a doctor's degree.
She failed to finish the assignment in time, and she was worried about it.
There is a man at the reception desk who seems very angry and I think he means to make trouble.
I’d like there to be a room of my own.
I don’t want there to be any misunderstanding between us.
b) 不定式作宾语还常用在下面结构中“主语+动词+it+形容词+不定式”
We found it impossible to get everything ready in time.
I think it important to learn English well in college.
使用这种句型的常见动词有：believe, consider, declare, feel, find, guess, imagine, prove, realize, suppose, think,等。
c) “wh-+不定式”结构
不定式前加一个疑问代词(what, which, who, whom, whose)或疑问副词(where, when, how, why)，以及连词whether构成特殊的不定式短语，其作用相当于一个分句，这样的不定式短语常在某些动词后面作宾语。常见的可以接这种不定式短语的动词有：know, see, decide, tell, ask, consider, discover, explain, find out, forget, guess, hear, imagine, inquire, learn, observe, perceive, remember, think, understand, wonder等。
I couldn't decide which book to choose.
I can tell you where to get this book.
They found it hard to decide whether to go swimming next Sunday or to visit Aunt Sally.
“wh-+不定式”结构除了作宾语外，还可以在句中作主语或者表语。
When to start the program remains undecided.
The question is how to put the plans into practice.
4)作状语
a) 不定式作状语表示目的。或用于so as (not) to和in order (not) to之后，来强调这种目的。
To learn a foreign language well, you must make painstaking efforts.
Mother saved every cent she could spare to pay for my schooling.
We must develop science and technology at high speed so as to raise scientific and cultural level of our country.
In order to get a high mark in Band 4, he did a lot of exercises both in grammar and reading comprehension.
b) 不定式表示结果，特别是在so...as to, such...as to, only to...以及too...to等结构中的不定式表示结果。
I rushed to the station as fast as I could, only to find the train already gone.
She left her hometown with her beloved man, never to return.
No one is too old to learn.
Would you be so kind as to carry the luggage for me?
c) 不定式常用来修饰形容词，构成下列词组：be able to, be afraid to, be apt to, be bound to, be certain to, be easy to, be eager to, be fit to, be likely to, be ready to, be sure to, be unable to, be unwilling to, be willing to等。
Chinese team is bound to win the World Cup this time.
It is likely to rain today, for it is very cloudy.
I am willing to help you with your homework, for we are friends.
5)作宾语补足语
a) 不定式常跟在下列动词之后作宾语补足语：ask, advise, allow, beg, cause, compel, command, enable, encourage, expect, feel, force, find, hear, have, inform, invite, let, make, mean, notice, order, permit, persuade, remind, require, request, teach, tell, urge, watch, warn, watch等。
Because of the complexity of the modern equipment, most offices require secretaries to have specified training.
He asked you to call him at ten o'clock.
The note reminds me to be careful whatever I do.
I'd never allow my children to behave like that.
b) 当不定式在let, make, have, hear, look at, listen to, feel, observe, watch, notice, perceive(感觉到)等动词后面作宾语补足语时，不定式不带to。
Whenever something is wrong with you, please do let me know.
I will have the students write a passage about Internet.
I saw my mother shed tears at the news that the neighbor girl got seriously hurt in a car accident.
It seemed so long before he heard the stone hit the water.
6)作主语补足语
带有宾语及宾语补足语结构的动词变为被动语态时，原来的宾语变为主语，宾补则变为主补。加主补的动词主要有：assume, believe, know, report, say, suppose等。
Mr. Brown is said to have left for Italy last week. (It is said that Mr. Brown left for Italy last week.)
Persons have been said to climb on roofs, solve mathematical problems, compose music, walk through windows and commit murder in their sleep.
He is reported to have won the 100-meter running race in the Olympic games.
7)作定语
a) 不定式作定语通常要放在其所修饰的名词或代词之后。
不定式常作下列名词的定语：attempt, ability, anything, chance, desire, determination, decision, effort, failure, intention, need, opportunity, plan, promise, pressure, right, tendency, time, way等。
His efforts to carry out the plan were successful.
I have no intention to go to the cinema with you.
There is no need to bother him with such trifles.
There is a tendency to write quite long sentences in commercial correspondence.
The pressure to compete causes Americans to be energetic, but it also put them under a constant emotional strain.
注意：不定式与其修饰的名词有动宾关系时，要根据句子的需要在不定式后加适当的介词，这个介词的选用取决于被修饰的名词或不定式本身的要求。
She is a very nice person to work with.
This is an important issue to talk about.
b) 由only, last, next,序数词或形容词最高级修饰的名词常用不定式作定语。
Mr. Zhang is always the first man to arrive at the office and the last man to leave.
I don't think he is the best one to do the work.
2.不带to的不定式(Bare Infinitive)
在下列词组后面的不定式不带to：would rather ...than(宁愿……也不), had better...(最好)，can't help but...(不得不), had rather...(宁愿)，cannot but...(不得不，必然)，may/might as well...(不妨)，let alone(更不用说), can do nothing but…(只能)。
You'd better return the books to the library on time. Otherwise, you will be fined.
I can't help but wish that nothing would go wrong.
They had never seen such delicious food, let alone eat it.
3.不定式的完成体
不定式的完成体表示不定式的动作在谓语表示的动作(状态)之前完成。
I am sorry to have kept you waiting for such a long time.
She seems to have read the book before.
The book is reported to have been translated into English.
He is said to have written a new book about business English.
The bank is reported in the local newspaper_______ in broad daylight yesterday. (2000年12月)
A) robbed B) to have been robbed
C) being robbed D) having been robbed
本句意为：据地方报纸报道，昨天这家银行在光天化日之下遭抢劫。
不定式的动作发生在主句动作之前，故选择不定式的完成体，答案为B)。to have been robbed在句子中充当主语补足语。
不定式完成体用在was/were, planned, intended, hoped, wished等之后，表示事实上并未实现的计划或行为;intend, wish, hope, mean, plan等动词的过去完成体与不定式一般式连用，也可表示事实上并未实现的行为。
The second novel was to have been completed by 1963, but two years later, the end was till nowhere in sight.
Japan hoped to have extended its power to whole Asia, but it didn’t succeed.
(=Japan had hoped to extend its power to whole Asia, but it didn’t succeed.)
We planned to have finished the work before supper.
(=We had planned to finish the work before supper.)
4.不定式的逻辑主语
在表示人物情况、特征等的形容词后面，常用of引导不定式逻辑主语，表示对逻辑主语的评价;如果形容词仅仅修饰不定式，与逻辑主语无关，则用for引导不定式逻辑主语。
It is very kind of you to have helped me.
It is foolish of him to have waste so much time in trivialities.
It is very important for us to hold a meeting as soon as possible.
It would be surprising for there not to be any objections to the proposal.
5.不定式与分词作定语的区别
根据非谓语动词表示的时间概念确定具体的非谓语动词形式(不定式、现在分词和过去分词)。涉及将来动作时用不定式;表示已完成的动作或状态，用过去分词;表示正在进行的动作或目前的状态时，用现在分词。
The problem to be discussed tomorrow is an urgent one.
(“讨论”是将来的行为，所以用不等式)
The problem discussed yesterday is an urgent one.
(“讨论”是完成了的行为，所以用过去分词)
The problem being discussed now is an urgent one.
(“讨论”是现在的行为，所以用现在分词)
6. 不定式作宾语补足语和现在分词作宾语补足语的区别
动词不定式只说明宾语的一个动作，而现在分词则说明宾语的动作正在进行。
Just then he heard someone sing in the next room. (在那时，他听见有人在隔壁房间里唱歌。)
Just then he heard someone singing in the next room. (在那时，他听见有人正在隔壁房间里唱歌。)
From:https://www.zgghmh.com/CET-4/20000691.asp