何时用不定式何时用分词和动名词Ⅰ.动名词和不定式做主语的相同点1.不定式短语和动名词做主语时，常常为了保持句子的平衡，用it来做形式主语，如：It is not easy to learn English well.It is no use asking him for help.2.如果主语用了不定式,表语和宾语也要用不定式;主语用了动名词,表语和宾语也要用动名词,这在英语中称为对称性,如:To see is to believe. = Seeing is believing. (眼见为实)(85)It is better to stay home than _A__ out.A. to goB. goingC. goD. goneⅡ.动名词和不定式做主语的不同点1.不定式做主语常表示具体的某一动作;表示泛指或一般的抽象的概念时,多用动名词,如:(92)_B_ is a good form of exercise for both young and old.A. The walkB. WalkingC. To walkD. Walk2.在”There/It +be+ no use(good/help/need)”之后常用动名词做主语,而少用不定式,如:It /There is no use crying over the split milk.It /There is no help taking this medicine.★但是如果该句型中用了for引出不定式的逻辑主语,就只能用不定式,如:(93) There is no good for us to do that exercise.3.不定式可以和when, where, how, what, whether等连用,在句子中做主语,宾语和表语,而动名词则不能,如: How to solve the problem is a hot potato.Where to put the box seems hard to decide.Whether to do it hasn’t been decided.What to do next is up to you.时态、语态及其基本用法A. 不定式1．不定式的时态、语态有三种形式：①一般式，如，to write / to be written ;②进行式，如，to be writing / to have been writing;③完成式，如，to have written / to have been written 。
2．基本用法①不定式的一般式，表示的动作与谓语动词表示的动作同时发生，有时也发生在谓语动词表示的动作之后。
如：He seems to know this.(=It seems that he knows this.)I hope to see you again.(=I hope that I’ll see you again.)He doesn’t like to be laughed at. (=He doesn’t like that he is being laughed at.)②不定式的进行式，表示不定式的动作正在进行，与谓语动词表示的动作同时发生。
如：When the mother went into the room, the boy pretended to be sleeping. (=… , the boy pretended that he was sleeping .)I heard the English song being sung by him. (=I heard that the English song was being sung by him.)③．不定式的完成式，表示的动作发生在谓语动词表示的动作之前。
如：I am sorry to have kept you waiting so long. (= I am sorry that I kept you waiting so long. )Having been well written, the book was translated into many languages. (=Because it had been well written, …)B. 分词1.只有现在分词才有时态和语态形式的变化。
现在分词的时态、语态有两种形式：①一般式，如，doing / being done;②完成式，如，having done / having been done 。
2.基本用法①现在分词的一般式，表示的动作与谓语动词表示的动作同时发生，或在谓语动词表示的动作之前发生。
如：The boys and girls went to school, singing and dancing all along the way .They all jumped with joy ,hearing the good news.The girl could not help crying ,being criticized in the public.②现在分词的完成式，表示的动作在谓语动词表示的动作之前已发生，常用作时间状语或原因状语。
如：Having done the work very well ,he was praised by the teacher.Having been shot by a bullet, the man fell down dead.C．动名词1.动名词的时态、语态有两种形式：①一般式，如，writing / being written ; ②完成式，如，having written / having been written 。
2.基本用法①动名词的一般式不表示动作正在进行，但作具有状语性质的宾语时，动名词一般式表示的动作与谓语动词的动作同时发生。
如：The teacher is busy collecting the stude nts’ exam papers.②动名词的完成式在作具有状语性质的宾语时，表示的动作在谓语动词的动作之前已发生。
如：He was praised by the teacher for having done the work wonderfully.He felt so sorry for not having been given the chance to work in the company.三、不定式、分词、动名词的词性特征和句法功能1．不定式既有动词的一些特征，又具有名词、形容词、和副词的句法功能，因此，不定式在句中作：主语、表语、宾语、宾语补足语、定语、状语、独立成分。
2．分词即具有动词的一些特征，又具有形容词和副词的句法功能，因此，不定式在句中作：表语、宾语补足语、定语、状语、独立成分。
3．动名词，顾名思义，既有动词的一些特征，又具有名词的句法功能，因此，动名词在句中作：主语、表语、宾语、定语、同位语。
四、不定式、分词、动名词句法功能用法的比较1. 从上面不定式、分词、动名词的词性特征和句法功能中，我们不难看出它们在句子中的作用既有较多的共性，又有稍微的差异：a.相同的句法功能：①不定式、分词都能作：表语、定语、宾语补足语、状语、独立成分；②不定式、动名词都能作：主语、表语、宾语；③不定式、分词、动名词都能作：表语、定语。
b.不同的句法功能：不定式可作：同位语。
如：That was her desire, to go to America for further study after finishing middle school.2.正确运用好不定式、分词、动名词⑴．不定式、分词、动名词作表语时的比较。
请看例句：①My job is to teach the students English.②My job is teaching the students English.③My job is interesting.④I am interested in English.辨析：A. ①不定式与动名词作表语一般可以互换使用，在意义上没有多大差别，且其形式差异较大，很容易把握, 如上面例句的①②；②现在分词和动名词同形，在作表语时，学生不易识别。
分辨方法有三：a）分词作表语说明主语的性质特征，回答how的问题；不定式和动名词作表语则说明主语的内容，回答what 或doing what 问题, 如：My job is interesting.(=How is my job?) ; My job is teaching ( to teach ) the students English. (=What is my job?); b）动名词具有名词的句法功能，也能做主语，我们不妨把句中的表语转换成主语，如果句子成立，则是动名词作表语，否则是现在分词作表语。
如：My job is teaching the studentsEnglish→Teaching the students English is my job.显然句子是成立的，原句中的teaching是动名词；c）分词具有形容词的句法功能，那么，我们可以在作表语的分词前加very(extremely ,fairly ) 等副词，如果句子成立，则动词—ing 形式作表语的是现在分词。
如上面例句：My job is interesting. 可改为：My job is very interesting. 显然句子是成立的，句中的interesting 是现在分词。
B．确定用现在分词还是用过去分词作表语的方法有：现在分词作表语多表示主语所具有的某种特征；过去分词作表语多表示主语所处的状态。
情感动词的v.-ing 形式表示具有某种特征；情感动词的v.-ed形式多表示引起某种情感。
一般地说，事物具有某种特征，而人才具有某种情感，因此，分词作表语的句式可归纳为：a）Sb.﹢be ﹢v-ed ; b）Sth. ﹢be﹢v-ing.如：a. I am very excited ; b. The news is very exciting.但有一个词例外，那就是missing 。
例如“那个男孩不见了”，我们不能说“The boy is missed .”，而是“The boy is missing.”⑵．不定式、分词、动名词作定语时的比较A．作前置定语现在分词、过去分词、动名词可以作前置定语，而不定式不能作前置定语。
如：① a developing country , a developed country ; boiling water，boiled water ; ②building materials , a swimming pool ; the sleeping boy , the coming new year 。