⒈不定式作主语
    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.
    (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.
    ②It takes sb+some time+to do
    How long did it take you to finish the work?
    ③It＋be＋形容词＋for sb＋to do
    It is difficult for us to finish writing the compositionin a quarter of an hour.
    ④It＋be＋形容词＋of sb＋to do
    It is stupid of you to write down everything the teachersays.
    ⑤It 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'skind of you to help me with my English.=You are kind to helpme with my English.
    ⒉动名词作主语
    Learning without practice is no good.
    动名词作主语时，也常用It句式。如：
    ①It's +no good(no use,fun,a pleasure,a waste of time)+doing…
    It's no good reading in dim light.
    It's no use sitting here waiting.
    ②It's＋形容词＋doing
    It's dangerous swimming in the sea in windy days.
    这样用的形容词有expensive,nice,tiring等，但important,necessary则不适用于这种结构，应用不定式 代替，如：It'simportant for you to keep fit.
    ③There is no+doing
    There is no saying what will happen next.
    在这一结构中，动名词后常带宾语，相当于"It's impossible to…"结构。
    ⒊动词不定式和动名词作主语的区别
    ①不定式作主语经常表示具体动作，常与特定的动作执行者联系在一起；而动名词作主语经常表示抽象动 作，经常不与特定的动作执行者联系在一起。如：
    It's no good eating too much fat.
    It's no good for you to eat so much fat.
    ②动名词结构作主语，可以用名词或代词属格形式作逻辑主语。如：
    It's no use your pretending that you didn't know the rules.
    二、作宾语
    ⒈不定式作宾语
    ①以下动词后，只能跟不定式作宾语。如：agree,ask,aim,arrange,choose,decide,demand,expect,fail ,help,hope,lean,long,
manage,offer,plan,prepare,pretend,promise,refuse,wish等，这些词大部分可接th at引导的从句。如：
    I decided to ask for my money back.
    I decided that I would ask for my money back.
    When our visit to the farm was over,we expected to startback on foot.
    When our visit to the farm was over, we expected that wewould start back on foot.
    ②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 foreignlanguage well.
    He feels it his duty to help the poor.
    ③介词but,except,besides+to do(do)
    在这种句型中，如介词前有动词do，后面应接不带to的不定式；如无do，则接to不定式，即带do不带to， 带to不带do。如：
    The enemy soldiers had no choice but to give in.
    On Sunday afternoon I had nothing to do but watch TV.
    ⒉动名词作宾语
    ①以下动词后，只能接动名词作宾语，如：admit,appreciate,consider,delay,enjoy,finish,keep,imag ine,mind,miss,practise,resist,risk,save,suggest,don't mind,give up,insist,on, put off等。如：
    I suggest spending our summer vacation in a seaside town.
    You must give up smoking, for it does too much harm toyour health.
    ②动名词作介词的宾语
    I should go to attend the birthday celebration instead of staying at home.
    What about inviting Li Jun to make a speech?
    动名词前的介词有时可以省略，如：have difficulty(in)doing,have no trouble(in)doing,lose no ti me(in)doing,prevent/stop…(from)doing,there is no use(in)doing等。
    ⒊部分动词后面，既可接动词不定式，也可接动名词作宾语，意义不变。如：begin,continue,start,hat e,like,love,need,require,want等。
    在need,require,want后接-ing形式，表示被动意义，也可接不定式，但要用被动形式，如：Your handwr iting needs improving(tobe improved). hate,love,like接不定式表示特定的未来事件，接动名词表示目前 正在进行的活动或一般的行为。
    在下列情况下，一般要用不定式：
    ①hate,like,love前有would(should)时，如：I'd like to have a cup of coffee.
    ②当谓语动词begin,continue,start等是进行式时，如：Thestudents are starting to work on the di fficult maths problem.
    ③begin,continue,start与know,understand等状态动词连用时，如：I soon began to understand what
    was happening.
    ⒋advise,allow,encourage,forbid,permit等动词后接动名词作宾语，或带不定式作宾语补足语。如：
    Our teachers don't permit our swimming in the lake.
    Our teachers don't permit us to swim in the lake.
    ⒌部分动词后接不定式或动名词时，意义差别较大，应根据句子语境选择使用。
    ①forget,remember,regret后接不定式，表示现在或未来的动作，接动名词表示动作已经发生。如：
    Don't forget fo post the letter for me.
    Have you forgotten meeting her in Beijing Airport?
    Remember to close the windows before you leave.
    I remember writing him a letter a year ago.
    We regret to tell you that all of you are not invited toattend the meeting.
    They regretted ordering these books from abroad.
    ②mean to do 打算做某事
    doing 意味着……
    I meant to catch up with the early bus.
    This means wasting a lot of money.
    ③try to do 设法尽力做某事
    doing 试着做某事
    You should try to overcome your shortcomings.
    Try working out the physics problem in another way.
    ④stop to do 停下一件事去做另一件事（不定式作目的状语）
    doing 停止做某事
    On the way to the airport,I stopped to buy a paper.
    You'd better stop arguing and do as you are told.
    ⑤can't help doing 禁不住……
    to do不能帮助干……
    They couldn't help jumping up at the news.
    Sorry I have lots of work to do.So I can't help to make up the room for you.
    ⑥go on to do 做不同的事或不同内容的事
    doing 继续不停地做某事，指同一动作的继续
    He went on to talk about world situation.他接着又谈了世界形势。
    We'll go on fighting so long as there is oppression inthe world.
    ⑦leave off to do 离开某地去干什么（目的状语）
    doing停下某事
    It's time to leave off talking and to start acting.
    They left off to go fishing.
    三、做表语
    不定式作表语表示具体动作或将来动作；动名词作表语表示抽象的一般行为。
    ①To be kind to the enemy is to be cruel to the people.
    ②My chief purpose is to point out the difficulties ofthe matter.
    ③What I would suggest is to put off the meeting.
    当主语和表语都是不定式时，其含义一是条件，一是结果（例①）。当主语是aim,duty,hope,idea,mista ke,plan,purpose,suggestion等为中心词的名词词组（例②）时，或以what引导的名词性分句（例③），不定 式说明主语的内容。
    ④Our work is serving the people.
    ⑤What he likes is taking a walk after supper.
    ⑥The story told by Mr.Wang is interesting.
    ④⑤句动名词作表语，与主语部分可以转换，如Serving thepeople is out work,而⑥句中是现在分词作 表语，说明主语的性质、状态，现在分词具有形容词的各种特征，另外，动名词作表语还应与进行时态区别开 来。
    四、作定语
    ⒈不定式作定语
    不定式在句中作定语，置于被修饰的名词或代词之后。如：
    ①The next train to arrive is from Washington.
    ②Have you anything to be taken to your sister?
    ③Do you have anything to say on the question?
    ④Would you please give me some paper to write on?
    ⑤My wish to visit France has come true at last.
    不定式短语作定语和被修饰词之间表示以下关系：
    (1)表示将来的动作（例①）。
    (2)与被修饰词之间有动宾关系，如是不及物动词，则需加介词（例④）。
    (3)与被修饰词之间有动宾关系，同时与句中其它词之间又有逻辑上的主谓关系时，尽管有被动含义，却仍 用主动语态（例③）；如只有动宾关系，而无逻辑上的主谓关系，则需用被动语态（例②）。
    (4)不定式作定语时，一般可转换为定语从句，例①to arrive=that will arrive。
    ⒉动名词作定语
    ①This passage can be used as listening materials.
    ②The reading room of our school library can hold 800people.
    ③All moving bodies have energy.
    ①②句动名词作定语说明一种性能，即：用来……的；第③句为现在分词作定语，单个分词作定语常置于 被修饰词之前，与被修饰词之间，可构成逻辑上的主谓关系，分词短语作定语常置于被修饰词之后。如：The man standing at the school gate is Professor Hua.
    五、不定式作补足语
    ⒈作宾语补足语
    一些及物动词除要求按宾语外，有时还需要有宾语补足语，说明宾语的行为、状态、特征，这时意思才相 对完整。
    (1)常要求不定式作宾补的动词有：allow,ask,advise,beg,cause,drive（强迫），encourage,expect,fo rbid,force,get,would like(love,hate),order,permit,persuade,teach,tell,want,warn,wish等。如：
    ①Would you like me to give your regards to Mary?
    ②I want you to understand the whole passage clearly.
    (2)部分动词后常接to be＋形容词、名词短语等形式，有时to be可省略，如：believe,consider,discov er,find(=consider),feel(=think),imagine,judge,know,prove,think,suppose,see(=understand),understa nd等。
    ①We all believe John(to be)honest.
    ②I consider him(to be)one of the best biology teachers of No. 1 Middle School.
    但当不定式是完成式时，to不能省略，如：We consider him tohave been foolish.
    (3)感觉动词和使役动词后用作宾补的不定式须省略to。
    ①I didn't hear anyone say anything about it.
    ②They make the students do too much homework every day.
    这种句式在变为被动语态时，to不能省略，如第②句：Thestudents are made to do too much homework
    every day.
    (4)help,know后面的"to"可有可无。如：
    Would you please help me(to) fill in the taxform?
    I've never known her(to)be late before.但：He was known to have been to France before.
    (5)部分短语动词后，常接不定式作宾补，如：
    You may depend on them to be there early.
    The Party calls on us to increase production and practise economy.
    常这样用的短语动词有：ask for,care for,call on,count on,depend on,wait for,long for（渴望）， prepare for,wish for等。
    ⒉作主语补足语
    不定式作主语补足语，和主语构成一种逻辑上的主谓关系。如：
    ①He was not allowed to enter the classroom for being late.
    ②The young university student is considered to have great promise.
    六、不定式作状语
    ⒈作目的状语
    (1) ①I stayed there to see what would happen.
    ②Henry has decided to go to the hospital to be examinedby the doctor.
    (2)有时为了强调，不定式前可加in order或so as。如：
    Bob took down my telephone number so as(in order)not toforget it.
    有时为强调目的状语可把in order to或不定式置于句首，但so as to不能这样用。在这种句式中不定式部 分可转换为so that,in order that,成为目的状语从句，如：
    I stayed there so that (in order that)I could see whatwould happen.
    (3)在部分表示感情色彩的形容词、过去分词或动词之后可接不定式，如：astonished,glad,happy,laugh ,pleased, sad,smile,sorry,surprised等。
    ①We are glad to hear the news.
    ②I was surprised to see that a three-year-old baby could write so well.
    在部分形容词后接不定式，用主动形式表示被动意义，这种句型中的主语是不定式的逻辑宾语。如：The question raised by thestudent is difficult to answer.
    The room is really comfortable to live in.
    常这样用的形容词有：comfortable,easy,dangerous,difficult,expensive,fit,impossible等。
    ⒉作结果状语
    We came home after our holiday to find our garden neat and tidy.
    不定式作结果状语还常用在下列句式中。如：
    ①so…as to;such…as to
    I'm not so stupid(a fool) as to put it in writing.我不至于愚蠢到会把它写下来。
    I'm not such a stupid fool as to put it in writing.
    ②enough…to
    The speed is high enough for us to catch up with thefirst liner.
    ③only to
    Jane hurried back only to find her mother dying inthehospital.
    ④too…to
    I'm too tired to stay up longer.
    但在下列结构中，too…to并非是“太……而不能……”之意。如：
    ①I'm only too glad to have passed the exam.考试及格我太高兴了。（too修饰glad to have…，相当 于very）
    ②We have too much to learn.我们要学的太多了（不定式作定语）。
    ⒊不定式短语还可作独立成分，用于句首、句中或句末。如：
    To tell the truth,the play was a great disappointment tome.
    常见的短语有to be exact（确切地说），to begin with（首先），to do him justice（说句对他公道的 话），to be sure（真的）等等。
    七、动词不定式、动名词的其它用法
    ⒈疑问词＋不定式结构
    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.
    ②Mr. Smith didn't know whether to leave or stay there.
    ③I asked Professor Xu how to learn English well.
    ④The question was where to get the medicine needed.
    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。
    ⒉动词不定式的时态、语态
    (1)时态
    ①一般式：动词不定式一般式表示的动作发生在谓语动词之后，有时表示同时发生。如：
    I hope to become a university student this year.（tobecome发生在hope之后）
    We often hear Dick play the piano in the next room.（play和hear同时发生）
    ②完成式：表示的动作发生在谓语动词所表示的动作之前。如：
    I'm sorry to have kept you waiting.
    We are too young to have seen the old society.
    ③进行式：表示的动作与谓语动词同时发生。如：
    The teacher happened to be correcting our papers when Icame in.
    They seemed to be discussing something important.
    (2)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）
    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）
    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 t ake care of (to be taken care of).但有时两种形式表达的意思不同，如：These is nothing to do now.( We have nothing to do now.) There is nothing to be done now.(We can do nothing now.)
    ⒊动名词的时态、语态
    (1)时态
    ①一般式：动名词的一般式所表示的动作可以是泛指，也可与谓语动词同时发生，或发生在谓语动作之前 、之后。如：
    We are interested in collecting stamps.
    I shall never forget seeing the Great Wall for the firsttime.
    We are not afraid of dying.
    ②完成式所表示的动作发生在谓语动词之前。如：
    Lmagine having travelled on the moon.
    We were praised for having finished the work ahead of time.
    (2)被动语态
    ①如果动名词的逻辑主语为动名词所表示的动作的承受者，动名词要用被动语态，动名词的被动语态有一 般式与完成式之分。如：
    The young man came in without being noticed.
    He prided himself on having never been beaten in class.
    ②有些动词后的动名词用主动形式，但表示被动意义。如：
    The bike needs repairing.
    If a thing is worth doing, it is worth doing well.
    ⒋在口语中，为避免重复，常用"to"代替不定式结构，有时甚至可以把to省略。如：
    ①—Did you go to visit the Great Wall?
    —No,I wanted to,but there wasn't enough time.
    ②—Would you like to come to a party?
    —I'd love to.
    ③—Don't make any mistakes in your homework,will you?
    —I'll try not to.
    ④—Try to be back by 12,won't you?
    —OK,I'll try.
    另外，be going to,ought to,used to等也常用于这一结构中。
    ⒌在why引起的问句中，省略"to"。如：
    Why spend such a lot of money?
    Why not wait for a couple of days?
    ⒍当两个或更多作用相同的不定式并列使用时，只在第一个不定式前加"to"。如：
    It's quite necessary for us to read more and have morepractice.
    ⒎"to"在下列短语中是“介词”，后接动名词或名词形式。如：
    devote…to,face up to（勇敢地面对），look forward to（盼望），object to（反对），take to（养 成习惯，对……感兴趣；开始从事某种活动），be used to（习惯于）等。