英语中动词的主要功能是做谓语,而动词不定式不能作谓语,所以叫动词的非谓语形式.动词不定式有两种形式,一是带to的不定式,一是不带to的不定式,后者都由动词原形构成. 不定式有动词的性质,有语态和时态.如：
一般式主动语态：to write 被动语态：to be written
进行式：to be writing
完成式：to have written 完成式被动语态：to have been written
动词不定式的动词性质还表现在可带宾语和状语组成不定式短语.例：Bill asked me not to say his name aloud.
(比尔要我别大声叫他的名字.有宾语和状语.）
不定式也有名词的性质,表现在可以在句中做主语、宾语、表语等.如：
To be content with little is true happiness.知足常乐.（主语）
She wants to improve her biology.她想提高她的生物学水平.（宾语）
He was the right man to marry Jane. 他正是要娶简的人.（定语）
动词不定式在句子中非常活跃,除了谓语之外所有的成分都能担当,位置往往灵活多变,要根据句子来具体分析,这大概是称为“不定”的原因吧.
(一)动词不定式：（to）+do,具有名词、形容词、副词的特征.
否定式：not + (to) do
（1）一般式：不定式的一般式所表示的动作与谓语动词动作同时发生或发生在谓语动词动作之后.
例如：
I'm glad to meet you.
He seems to know a lot.
We plan to pay a visit.
He wants to be an artist.
The patient asked to be operated on at once.
The teacher ordered the work to be done.
（2）进行式：不定式的进行式所表示的动作与谓语动词动作同时发生,例如：
The boy pretended to be working hard.
He seems to be reading in his room.
（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 regretted to have told a lie.
I happened to have seen the film.
He is pleased to have met his friend.
2．不定式的句法功能：
（1）作主语：
To finish the work in ten minutes is very hard.
To lose your heart means failure.
动词不定式短语作主语时,常用it作形式主语,真正的主语不定式置于句后,例如上面两句可用如下形式：
It is very hard to finish the work in ten minutes.
It means failure to lose your heart.
常用句式有：1、It+be+名词+to do.2、It takes sb.+some time+to do.3、It+be+形容词+of sb +to do.4、It+be+形容词+for sb.+to do.常用careless,clever,good,foolish,honest,kind,lazy,nice,right,silly,stupid,wise,等表示赞扬或批评的形容词,不定式前的sb.可作其逻辑主语.
（2）作表语：
Her job is to clean the hall.
He appears to have caught a cold.
（3）作宾语：
常与不定式做宾语连用的动词有：want, hope, wish, offer, fail, plan, learn, pretend, refuse, manage, help, agree, promise, prefer, 如果不定式（宾语）后面有宾语补足语,则用it作形式宾语,真正的宾语（不定式）后置,放在宾语补足语后面,例如：
Marx found it important to study the situation in Russia.
动词不定式也可充当介词宾语,如：
I have no choice but to stay here.
He did nothing last Sunday but repair his bike.
动词不定式前有时可与疑问词连用,如：
He gave us some advice on how to learn English.
（4）作宾语补足语：
在复合宾语中,动词不定式可充当宾语补足语,如下动词常跟这种复合宾语：want, wish, ask, tell, order, beg, permit, help, advise, persuade, allow, prepare, cause, force, call on, wait for, invite.
此外,介词有时也与这种复合宾语连用,如：
With a lot of work to do, he didn't go to the cinema．
有些动词如make, let, see, watch, hear, feel, have等与不带有to的不定式连用,但改为被动语态时,不定式要加to, 如：
I saw him cross the road.
He was seen to cross the road.
（5）作定语：
动词不定式作定语,放在所修饰的名词或代词后.与所修饰名词有如下关系：
①动宾关系：
I have a meeting to attend.
注意：不定式为不及物动词时,所修饰的名词如果是地点、工具等,应有必要的介词,如：
He found a good house to live in.
The child has nothing to worry about.
What did you open it with?
如果不定式修饰time, place, way,可以省略介词：
He has no place to live.
This is the best way to work out this problem.
如果不定式所修饰名词是不定式动作承受者,不定式可用主动式也可用被动式：
Have you got anything to send?
Have you got anything to be sent?
②说明所修饰名词的内容：
We have made a plan to finish the work.
③被修饰名词是不定式逻辑主语：
He is the first to get here.
（6）作状语：
①表目的：
He worked day and night to get the money.
She sold her hair to buy the watch chain.
注意不定式放句首时,逻辑主语与句子主语要一致：
wrong：To save money, every means has been tried.
right：To save money, he has tried every means.
wrong：To learn English well, a dictionary is needed.
right：To learn English well, he needs a dictionary.
②表结果(往往是与预期愿望相反的结果)：
He arrived late only to find the train had gone.
I visited him only to find him out.
③表原因：
They were very sad to hear the news.
④表程度：
It's too dark for us to see anything.
The question is simple for him to answer.
（7）作独立成分：
To tell you the truth, I don't like the way he talked.
（8）不定式的省略：保留to省略do动词.
If you don't want to do it, you don't need to.
（9）不定式的并列：第二个不定式可省略to.
He wished to study medicine and become a doctor.