1．1 作主语
To talk to him is a great pleasure.
To die for people is worthwhile.
* 动词不定式作主语时，往往放在谓语动词的后面，而用 it做形式上的主语。上面两句可改写成：
It is a great pleasure to talk to him.
It is worthwhile to die for people.
1．2 作宾语：不定式作宾语有两种，一是"及物动词 + 带 to 的不定式"；另
一种是"动词+疑问词+ to 不定式"
1) "及物动词 + 带 to 的不定式"。这类后面跟动词不定式的及物动词有：agree, arrange, afford,
offer, manage, endeavor, promise, swear, seek, intend, aim, undertake,
pretend, decide, determine, expect, long, hope, wish, desire, claim,
learn, fail, refuse, decline … 等。
He decided to make some changes in his plan.
Learn to walk before you run.
He wishes to be an engineer.
The Exchange House offers to provide coffee to customers.
They have arranged for us to visit the Great Wall.
2) "动词 + 疑问词 + to 不定式"。这类动词常见的有：advise, decide, discuss, find
out, know, learn, remember, see, understand, wonder…等, 这类疑问副（代）词有what,
where, which, how, whether…等。
| He doesn't know |
how to go there.
when to meet his cousin.
whom (who) to visit.
which one to choose. |
| She cannot decide |
whether to go or to stay.
when to see a doctor.
how to solve this problem.
whom to ask for help.
where to put my bag. |
1．3 不定式作表语。一种是主语是不定式，表语也是不定式；另一种是主语以下列名词aim, duty, goal, hope, intent,
job, object, plan, problem, purpose, thing, wish等为中心词时，不定式表语对主语起补充、说明作用，
其结构往往是"主语 + be + to 不定式表语"。
1) 主语是不定式，表语也是不定式
To eat means to live.
To help others is to help yourself.
2) "主语 + be + to 不定式表语"
His wish is to buy a used car.
The most important thing in her life is to make money.
The purpose of this action is to catch students who take tests for others.
The object of his visit to the professor is to ask for advice.