非谓语动词是指在句子中不是谓语的动词。主要包括：
不定式-Infinitive动名词-Gerund分词-Participle非谓语动词除了不能独立作谓语外，可以承担句子的其他成分。
作名词的用法：
To teach is to learn.教书就是学习。（作主语）I like to teach. 我乐于教书。（作宾语）To teach is to learn.教书就是学习。（作补语）注意：当不定式作主语，或作使役动词- Factitive Verb(如：leave，get，keep，make，let，help，have)等的宾语时，常用it作形式主语或形式宾语，而把不定是放在后面。
It is never too late to mend.(作主语)I make it a rule to go out every day.(作宾语)此外：在作名词的不定式前出现what,how,where等疑问词时，可把它们看作名词短语。
例如：
He did not know how to swim.(=the way to swim)I wish I know where to go.(=the place for going)She scarcely knows what to do.(=what she ought to do)
作形容词的用法：
I have no friend to help me.我没有可以帮助我的朋友。He has no food to eat. 他没有可以吃的食物。注意：省略了关系代词的时候，介词就要一道不定式的后面。
例如：
We need a house to live in.(=in which to live)He had no money to buy food with.(with which to buy food.)3．完成式：不定式的完成式表示的动作发生在谓语动词动作之前
例如：
I happened to have seen the film. 我偶然看过这部电影。He is pleased to have met his friend. 他很高兴能遇上他的朋友。
作副词的用法：
①表目的：
He worked day and night to get the money. 他夜以继日地工作来赚钱。She sold her hair to buy the watch chain. 她卖掉了自己的头发来买那条表链。注意：不定式放句首时，逻辑主语与句子主语要一致
×：To save money,every means has been tried.√：To save money,he has tried every means. 为了省钱，他使出了浑身解数。×：To learn English well,a dictionary is needed.√：To learn English well,he needs a dictionary. 为了学好英语，他需要一本词典。②表结果(往往是与预期愿望相反的结果 意料之外)：常放在never only后
He arrived late only to find the train had gone. 他来晚了，只见火车已经走了。I visited him only to find him out. 我去拜访他，只见他出去了。③表原因：常放在形容词后面
They were very sad to hear the news. 他们听到这条新闻非常伤心。④表程度：
He is old enough to go to school.The question is simple for him to answer.
独立的不定式：与句中其他的词毫无语法关系，完全处于独立地位，用于修饰全句。
最好背下来To tell the truth,I do not like him.说实话，我不喜欢他。He is, so to speak, a book-worm.他可以说是个书呆子。To be frank with you, you are a man of weak will.坦率地说，你是个意志薄弱的人。To do him justice,he is not without some merits.说句公道话，他并非一无是处。He fell nearly 100 feet; but, strange to say, the fall did not kill him.他跌了将近100英尺;但是，奇怪的是，他并没有摔死。To make the matters worse, thick sleet was driving across the sea.更糟的是，厚厚的雨夹雪正飘过海面。He knows German and French,not to mention English.他懂德语和法语，更不用说英语了。
不定式的6种时态：
to doto be doingto be doneto have doneto have been doingto have been donefor Object InfinitiveFor him to fail to come would be fatal to our plan.(作名词)他不来对我们的计划是致命的。It was an easy thing for Lord Byron to be a great poet.（作名词）拜伦勋爵成为一位伟大的诗人是一件容易的事。She held the baby up for them to see.（作副词）她把孩子举起来让他们看。It is time for us to be up and doing.（作形容词）我们该起床做事了。需要敲黑板的内容可以省略to的不定式用法：
在动词see,hear,feel,make,let,have,observe,watch,notice,please等之后，省略to
例如：
I never saw anyone cry for joy before.I feel the cold air strike against my face.I made him come and sit beside me.I will have him come soon.I let him go back to his own house.Please have a seat.在had better,had best,had rather,would rather,could not but feel,do nothing but后习惯把to省略
例如：
I had better not work after you have tired yourself.It's nearly one,and we had best get a few hours' sleep.I had rather be a doorkeeper in the house of Lord,than dwell in the tents of wickedness.I would rather walk than run.I could not but feel sorry for what you had said.He does nothing but study.
