动词不定式概述
动词不定式指由to加上动词原形所构成的一种非限定性动词，但在有些情况下to 可省略。动词不定式在语法功能上可作主语、宾语、表语、定语和状语，如：
To ignore this dangerous signal would be a terrible mistake. (作主语)
He promised not to tell anyone else about it. (作宾语)
To treat comrades like enemies is to go over to the side of the enemy.(作表语)
The manager announced his intention to retire.(作定语)
The old man walked slowly on the ice in order not to fall down.(作状语)
不定式中的动词仍然具有动词的特性，可以有自己的宾语和状语，另外虽然动词不定式在语法上没有表面上的主语，但它所表达的意义是动作，这一动作的执行者就是该动词的逻辑主语，动词不定式要和它的逻辑主语在语态和时态上保持一致。
动词不定式的用法
1.不定式作主语
⑴原则上，可用动词不定式直接作句子的主语，通常用it作句子的形式主语，而把不定式移到后面，形成“It + 谓语… + 动词不定式…”的句型。如：
To catch the 5：00 bus early in the morning is not a good idea. It is not a good idea to catch the 5：00 bus early in the morning. To learn a foreign language well takes painstaking efforts. It takes painstaking efforts to learn a foreign language well.⑵若要说明不定式的动作的执行者，可以在不定式前面加上“for + 名词或代词”结构，或是“of + 名词或代词”结构。
It is a great pleasure for me to talk with him. It is very kind of you to help me .⑶可用于“of + 名词/代词 + 动词不定式”结构中作主语补足语的形容词是指那些可以表示人或事物的性质或品质的好坏的形容词。这些词如：careful， cruel， foolish， bold， brave， good， honest， kind， rude， polite， nice，stupid， selfish， silly， wicked， wise， thoughtful， wrong等。
2.不定式作宾语
不定式作宾语有两种情况：一是及物动词后直接跟带to的不定式，二是“动词+疑问词+带to的不定式”。
⑴“动词+带to的不定式”结构：常见的以不定式作宾语的动词有afford， appear， agree， ask， believe， decide， demand， desire， fail， hope， intend，learn， mean， offer， promise，refuse， wish等。如：
I can‘t afford to buy a house.⑵“动词+疑问代(副)词+不定式”结构：常见的这类动词有advise， decide， forget， know， learn， remember， see， tell， think， teach， wonder，discover， understand， guess， explain等。这类疑问代(副)词what， when， which， where， how， whether等(不包含why)。如：
She does not know how to go there. He will tell you how to deal with it.⑶在不定式作宾语时，有时我们会使用it作形式宾语，而把不定式放到后面。如：
They consider it inappropriate to have classes in the evening.
3.不定式作表语
不定式作表语时，主语通常是一个名词，不定式短语或者从句。
The boss‘s plan is to start building the skyscraper immediately. To work means to earn a living. What you have to do is to push the button.
4.不定式作定语
不定式作定语相当于一个定语从句，它修饰以下几类名词：
⑴由动词派生或转化而来的名词，它们所对应的动词通常有不定式作宾语。
Another attempt to climb the mountain will be made next month. In his speech he expressed his determination to achieve the goal.这类名词主要有：attempt， claim， decision， plan， promise， intention， need，resolution，tendency， wish， failure， threat等。
⑵由可能与不定式搭配的形容词派生出来的名词。
Her eagerness to learn a foreign language obvious.这类名词主要有：anxiety， patience， ability， willingness， ambition等。
⑶不定式作定语还可用来修饰代词something， anything， everything， nothing. She has everything to lose and nothing to gain.⑷由only， last， next， 序数词或形容词最高级修饰的名词后常用不定式作定语。Jim is the only student to answer the question correctly in his class. You are the last to undertake the blood test.⑸不定式常作为定语修饰there be结构的真实主语。
There is a big task to complete.⑹ 有些抽象名词。主要有：reason， right， time， effort， chance， way等。如： the time to go to school， the way to get to the clinic， the chance to win the game.
5.不定式作状语
不定式作状语相当于一个状语从句，表示形为的目的、结果或原因。
⑴表示目的。在这种情况下，不定式的逻辑主语通常也是全句的主语。
He stayed there to see what would happen.在强调不定式目的状语时，不定式前可加in order， so as等。如：
She asked me to drive her to the airport in order to catch the 5：00 plane. He moved the TV set to the meeting-room so as to let everyone watch it.
⑵用so……as to……， such (……) as to……， enough to……， too……to， only to等表示结果，如：
Have you got enough room to seat all of us?
She is too weak to join us in the outing.
⑶将不定式移到句首表示条件，如：
To hear the teacher better， you have to sit in the front. = If you are to hear the teacher better，……