不定式省略to的九种情况
一、使役动词后省略to的情况
在let, make, have等使役动词后用作宾语补足语的不定式必须省略to。如：My mother wouldn’t let me go to the film. 我妈妈不会让我去看电影的。I don’t like milk, but mother made me drink it. 我不喜欢牛奶，可是母亲强迫我喝。I would have him wait for me at the gate of the park. 我要他在公园门口等我。注意： 1. 当使役动词用于被动语态时，要补上在主动语态中省略的to(主要是指make，let和have很少用于被动语态)。2. force, oblige等虽然也表示2. force, oblige“使”，但它们后用作宾语补足语的不定式必须带to。如：He forced me to go with them. 他迫使我同他们一起去。The police obliged him to leave. 警方强迫他离开。
二、感觉动词后省略to的情况
在感觉动词后用作宾语补足语的不定式必须省略to。如：I watched her get into the car. 我看着她上了车。I saw the woman enter a bank. 我看见这个女人进了一家银行。We often hear her sing this song. 我们经常听到她唱这首歌。 Did you notice her leave the house? 她离开屋子你注意到了吗?注意：1. 这里所说的感觉动词主要包括see, hear, observe, notice, feel, watch等。但是它们用于被动语态时，其后的不定式必须带to。如：The woman was seen to enter a bank. 有人看见这个女人进了一家银行。但是，用于以上句型的动词notice 和watch和通常不用于被动语态。2. 类似地，动词look at和listen to后用作宾语补足语的不定式也不带to。如：We listened to the old man tell his story. 我们听这位老人讲述他的经历。3. 若动词feel后用作宾语补足语的不定式为 to be，则要带 to(其他情况不带 to)。如：They felt the plan to be unwise. 他们认为这个计划不明智。4. 若不定式为完成式，通常应带 to。如： I noticed her to have come early. 我注意到她来得很早。
三、动词help后省略to的情况
在动词help后用作宾语或宾语补足语的不定式可以不带to。如：Can I help (to) carry this heavy box? 我可以帮忙扛这个重箱子吗？Mother helped me (to) do my homework. 妈妈帮助我做作业。注意：1. 当 help 之后接一个较长的名词词组作宾语或当其中的不定式所表示的动作主语不直接参加时，不定式通常带to。如：Help the little boys at the back of the hall to carry the chairs out. 请帮大厅后面那些小男孩把椅子搬出去。These tablets will help you to sleep. 这些药片将对你的睡眠有帮助。2. 在当help用于被动语态时，不定式前的to不能省略。如：
The boy was helped by a woman to collect his scattered coins. 那男孩由一个妇女帮他捡拾散乱一地的钱币。
四、why (not)…后省略to的情况
在why (not)…?之后的不定式不能带to。如：Why go with him? 为什么要同他一起去？Why not ask the teacher? 为什么不去问问老师？Y: Why not try to expand your story into a novel? 你怎么不把你的故事扩展成小说呢?
五、动词know后省略to的情况
在宾语+不定式”中，不定式有时省略to，有时不省，可分以下两种情况讨论：1. 若 know 为现在式，该结构中的不定式只限于 to be，且其中的to，且其中的不能省略。如：I know him to be ill. 我知道他病了。We knew her to be honest. 我们知道她是诚实的。2. 若know为完成式或过去式，则该结构中的不定式可以用除 to be外的其他动词，且此时其中的to可以省留。如：I never knew him (to) do such a thing. 我从未听说他会干那种事。We’ve never known him (to) tell a lie. 我们从未听说他撒过谎。I’ve never known it (to) snow in July before. 我从未听说过有七月下雪的事。但在被动语态中不定式前的to不能省略。这样用的know不仅仅表示一般意义的“知道”，而是表示一种经历，因此常译为“曾……过”(用于肯定句时)用于肯定句时或“(从来)没有……过”(用于否定句时)。
六、介词except / but后省略to的情况
用作介词except, but宾语的不定式有时带to，有时不带to。其大致原则是：若其前出现了动词 do，其后的不定式通常不带 to；若其前没有出现动词 do，则其后的不定式通常带 to。如：I had no choice but to wait. 除了等，我没有别的选择。He wanted nothing but to stay there. 他只想留在那儿。It had no effect except to make him angry. It had no effect except to make him angry. 除惹他生气外，没产生任何效果。She can do everything except cook. 除了做饭之外她什么都会。
七、主语带do表语省略to的情况
的情况当主语部分有动词do的某种形式时，用作表语的不定式可以省略to。如：All you do now is (to) complete the form. 你现在要做的只是把这张表填好。The only thing to do now is (to) go on. 前进是现在唯一的出路。What I’ll do is 我要做的就是告诉她真相。
八、并列不定式省略to的情况
当两个或多个作用相同的不定式并列时，通常只需在第一个不定式前用to，其余不定式前的to可以省略。如：He told me to stay there and wait for him. 他叫我在那儿等他。I’m really puzzled what to think or say. 我真不知该怎么想怎么说。It is easier to persuade people than (to) force them. 说服人容易，强迫人难。但是，如果两者有对比关系，则后面不定式前的to不可省略。如：To try and fail is better than not to try at all. 尝试而失败总比不尝试好。
九、省略不定式是否保留to
在一定的上下文中，为了避免重复，有时不定式可以省略，但通常保留不定式符号to。如：I shall go if I want to. 如果我想去就去。“Don’t be late.” “I’ll try not to.” “不要来晚了。”“我尽量不来晚。”Don’t go till I tell you to. 等我叫你走你再走。注意：1. 若被省略的不定式为to be短语，则通常应保留to be。如：He is not the man he used to be. 他已不是原来的那个样子了。2. 有时省略不定式时，同时也可省略to。如：She may go if she likes (to). 她想去就可以去。
1)不定式做主语一般表示具体的某次动作。
【例如】To complete the 30storied building in one year was
quite a difficult task.
To do that implies taking responsibility.
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。
【例如】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
【例如】It is essential to reserve a table in advance of Christmas Eve. hard to put my hopessintoswords.
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 (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)不定式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, task, purpose, duty, job等。
【例如】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.
3)不定式作宾语不定式常在下列动词后面作宾语：
afford,agree,apply,arrange,ask,attempt,beg,begin,care,
【例如】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 doctor’s degree.
The room is designed to be my study,but now it has to be used as a bedroom for the children.
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( )trouble. (CET-4 1997, 1)
A) making B) to make C) to have made D)shavingsmade
mean后面一般加不定式，所以A和D可以排除。根据句意，他看起来非常生气，他想找麻烦，不定式的动作还没有实施， 应用一般式，因此答案为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。
【例如】 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。
4)“wh�word＋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，其作用相当于一个从句，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，等
【例如】 I couldn’t decide which book to choose. I can tell yous wheresto get this book.They found it hard to decidewhether to go swimming next Sunday or to visit Aunt Sally in New Year.“wh�word+不定式”结构除了作宾语外，还可以在句中作主语或者表语。
【例如】When to start the program remains undecided.The question is how to put the plansintospractice.
5)不定式作状语
a)不定式作状语表示目的。或用于so as和in order之后，来强调这种目的。
【例如】 To learn a foreign language well, you must make painstakingefforts. Mother saved every cent she could spare to pay for my schooling.We must develop science and technology at high speedso as to raise scientific and cultural level of our country.Insgroups to get a high mark in Band 4, he did a lot ofexercises 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。
【例如】 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 readyto, be sure to, be unable to, be unwilling to, be willing to等。
【例如】 Chinese team is bound to win the World Cup.It is likely to rain today, for it is very cloudy.I am willing to help you with your homework, for we are friends.
6)不定式作宾语补语不定式常跟在下列动词之后作宾语补语：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。
【例如】Because of the complexity of the modern equipments,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.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语时，不定式不带to.
【例如】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 neighborgirl got seriously hurt in a car accident.It seemed so long before he heard the stone hit the water.
7)不定式作主补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。
【例如】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, solvemathematical problems, compose music, walk throughwindows and commit murder in their sleep He is reported to have won the 100�meter running racein the Olympic games.
8)不定式作定语
a)不定式作定语通常要放在其所修饰的名词或代词之后。
不定式常作下列的名词定语：attempt, ability, anything,chance, desire, determination, decision, effort, failure,ntention, need, opportunity, plan, promise, pressure,right, tendency,time, way等。
【例如】His efforts to carry out the plan were successful.I haveno intention to go to the cinema with you.There is no need to bother him with such triflesThere is a tendency to writer quite long sentencesin commercial correspondence.The pressure( )causes Americans to be energetic,but it also put them under a constant emotional strain.(CET-4 1997,6) A) to compete B) competingC) to be competed D)shavingscompeted
pressure后面应接后置定语，表示竞争的压力。单个分词作定语一般置于名词前，故B和D可以排除。C为不定式的被动式，而竞争和压力之间没有被动关系，因此正确答案为A。特别注意的是在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。
【例如】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。
【例如】
Mr. Zhang is always the first man to arrive at the officeand the last man to leave.I don’t think he is the bestone to do the work.
9)不带to的不定式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 helpbut...(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)。
【例如】 You’d better return the books to the libraryon 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.
http://learning.sohu.com/20060430/n243072167.shtml参考资料：http://page.hjenglish.com/page/37953/