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：
(1)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。
(2)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。
(3)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.
(4)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.
(5) 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.
(6) 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。
(1)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.
(2)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.