(1)做形式主语的代词：
不定式做主语, 通常用it充当形式主语, 把做主语的不定式短语后置。 如：
It took me only five minutes to finish the job.
To be frank, it is a great relief to have the task fulfilled in so short a time.
(2)引导逻辑主语的介词：
不定式的逻辑主语一般由介词for引导,但下列表示人的性格行为特征的形容词做表语时, 不定式的逻辑主语则由of引导：
absurd, bold, brave, courageous, careful, careless, clever, wise, foolish, silly, stupid, good, nice, kind, thoughtful, considerate, greedy, generous, honest, modest, polite, rude, cruel, selfish, lazy, wicked, wrong。如：
Experts say walking is one of the best ways for a person to stay healthy.
It’s clever of you to have invented such a device.
(3)不定式做主语补足语：
掌握常用不定式做主语补足语的句型。注意不定式表示的动作发生的时间，并采用相应形式。如：
said
reported
thought
be to do sth.
believed
known
supposed
Byron is said to have lived on vinegar and potatoes.
The bank is reported in the local newspaper to have been robbed in broad daylight yesterday.
2.不定式做宾语www.med126.com
(1)必须接不定式做宾语的动词：
掌握要求接不定式做宾语的动词：
agree, afford, aim, arrange, appear, ask, attempt, choose, claim, decide, desire, determine, expect, fail, guarantee, hope, endeavor, intend, long, mean, manage, offer, ought, plan, pledge, pretend, prepare, promise, proceed, prove, resolve, refuse, request, swear, tend, try, venture, wait, wish。如：
Even though the children pretended to be asleep, the nurses were not deceived when they came into the room.
注意:
1)有的动词要求特殊疑问词+不定式做宾语, 这类动词有：
consider, discover, explain, forget, guess, know, learn, observe, remember, see, tell, understand, wonder，如：
While still a young boy, Bizet knew how to play the piano well and as he grew older, he wrote operas, the most famous of which is Carmen.
2)如果该特殊疑问词在不定式中做介词宾语, 介词往往置于该特殊疑问词的前面。如:
The professor can hardly find sufficient grounds on which to base his argument in favor of the new theory.
Without facts, we cannot form worthwhile opinion for we need to have factual knowledge upon which to base our thinking.
(2)可以用不定式做宾语补足语(复合宾语)的动词：
下列动词可以用不定式做宾语补足语(复合宾语)：
advise, allow, ask, beg, cause, command, compel, enable, determine, encourage, expect, feel, find, force, hate, have, hear, help, inform, invite, let, like, make, mean, need, notice, notify, oblige, order, permit, persuade, remind, request, require, see, send, teach, tell, tempt, trouble, urge, want, warn, watch, wish。
注意：画线动词后面的不定式不带to。
如：
Because of the recent accidents, our parents forbid my brother and me to swim in the river unless someone agrees to watch over us.
