不定式的用法
不定式在句中可充当主语、表语、宾语、宾补、定语和状语等，下面谈一下不定式的易错点。
一、不定式作定语时.与其所修饰的名词具有动宾关系.同时又与句中的主语或宾语在逻辑上有主谓关系时.不定式用主动形式。例如:
(l)We still havemany difficulties to overcome
(to overcome与difficulties有动宾关系，同时与主语we在逻辑上有主谓关系。)
(2) I gave Tom abook to read.
(to read与book有动宾关系，同时与宾语Tom在逻辑上有主谓关系。)
如果不定式与其所修饰的名词虽有动宾关系，但与句中的主语或宾语在逻辑上没有主谓关系，不定式用被动形式。例如:
I’m leaving for Shanghai tomorrow. Do youhave anything to be taken to your son at shanghai University?
(to take与anything有动宾关系，但与句子主语you在逻辑上没有主谓关系。)
If there’s a lotof work____，I’m happy to just keep on until it isfinished.
A. to doB. to be doing C. done D. doing
解析:该题中不定式to do与a lot of work有逻辑上的动宾关系，与句子主语I在逻辑上有主谓关系，所以选A。
二、在“主语+系动词+形容词表语+带to不定式”的句型中，不定式用主动形式。如:
The question iseasy to understand.
【练习】I like getting up very early in summer. Themorning air is so good______.
A. to be breathedB. to breathe C.breathingD.beingbreathed
解析:考查形容词后跟不定式的主动形式表达被动意义。在“主语十be+形容词+不定式”句型中，若主语是不定式结构的逻辑宾语，这时应用主动形式表示被动含义。适合此结构的形容词最常用的有amusing，。heap，dangerous，difficult，easy，important，nice构成动宾关系。这里应用主动形式表被动意义，故答案为B。
三、不定式作表语，用来解释主语或主语从句中do的精确含义时，往往可以省略动词不定式符号to。例如:
(l)The firstthings he did was (to) go up to her trainer and thank her for all her helpduring her training.
(2)What we want todo now is (to) lie down and rest.
【练习】What I’11do is ______her the truth.
A.telling B.tellC.told D.tells
解析:此题主语从句有动词do，用作表语的不定式可以省略to，所以选B。
四、不定式作宾语补足语和主语补足语.(1)动词不定式作感官动词feel/see/notice/watch/hear/listento/observe和使役动词have/make/let等的宾语补足语时，动词不定式符号to省略，如果这些动词用于被动语态，其后的动词不定式符号to不能省略(但let，have除外)。(2)不定式在believe,consider, think, declare, find, guess, suppose等词后作补语。(3)在“Itis said/reported/believed that…等句型转化为“sb. Be said/reported/believedto do句型”中，不定式作补语。
例如:
(l)We often hear himsing the song at home.(=He is often heard to sing the song at home.)
(2)They knew her verywell. They had seen her grow up from childhood.(=They knew her very well She hadbeen seen to grow up from childhood.)
(3)we consider Tomto be one of the best students in our class.
【练习】The missing boys were last seen____near theriver.
A.Playing B.to bePlaying C.Play D.toPlay
解析:此题实际上感官动词跟动词ing形式(see sb.doing)表示动作同时发生，被动形式为sb.Be seen doing，所以答案为A。如果用to play，则表示play的过程已结束，逻辑不同。
【考例】Charles Babbage is generally considered_theflrst computer.
A.to inventB.inventingC.to have invented D.havinginvented
解析:由considersb. To do sth.排除B、D。此句中发明这一事实发生在前，不定式应该为现在完成时，所以选C。
【考例】一is Bob still performing?
一I’m afraid not. He is said_____the stagealready as he has become an official.
A.to haveleftB.to leaveC.to have been leftD. to be eft
解析:此题考查“sb.besadrepoted/believed to do"句型，由题意可知Bob已经离开舞台，所以
用不定式的完成时，故选A。
五、疑问词+to do作宾语时.常用主动形式。
常见动词有decide，know，consider，forget，learn，remember，show，understand，see，wonder，hear，find out，explain，tell等。例如:
They haven’t decidedwhen to leave for America.
【考例】I’ve worked with children before，so I know what____in my new job.
A.expectedB.to expect C.to be expectingD. expects
解析:该题考查疑问词+不定式的用法。“疑问词十带to的不定式”的结构，起名词词组的作用，该句中作know的宾语。疑问代词what与不定式to expect之间具有逻辑上的动宾关系，故最佳答案选b。
六、不定式作目的状语，例如:do what one can to do something，doeverything one can to do something, something be made use of to do something等。
(l)The teacher didwhat he could to save the boy!
此句子可改写成“The teacher did what he could (do) to savethe boy.”句中to save the boy作目的语。
(2)Every minuteshould be made use of to study hard.
此句可改写为“We should make use of every minute tostudy hard.”，句中to study hard作目的
语。
【考例】Time should be made good use of_our lessonswell.
A.learningB.learned C.to learn D.having learned
解析:该题考查不定式目的作状语，故选C。
七、如果在介词but, except之前有动词do的某种形式，其后不定式一般不带to.反之则必须带to。表示“不得不，只能”。如haveno choice but do，can do nothing but do等。还有一些含有but固定搭配短语如cannot help but，cannot choose but，cannot but，can but等，后面跟不带to的动不定式。例如:
She had no choicebut to go away.
【考例】She could do nothing but ____ to histeacher that he was wrong.
A.admit B.admitted e.admittingD.to admit
解析:该题中but前有do，其后不定式一般带to，所以选A。
【题例】Hearing the inspiring news，we couldn’t help but _____.
A.cheerB.cheeringC.to cheerD.cheered
解析:该题考查can nothelp but do固定结构，所以选A。
八、to作不定式符号与作介词的区别。例如
He has been used toliving here.Water can be used toirrigate fields.
【考例】--They are quiet，aren’t they?
-- Yes. They areaccustomed___at meals.
A.to talk B.to not talkC.to talking D.to not talking
解析:此题考查beaccustomed to doing，to为介词，所以选D。
