2) 功用: 不定式可以作主语 (a), 宾语 (b), 表语 (c), 定语 (d) 或是状语 (e).
a. To scold her would not be just.
b. We are planning to build a reservoir here.
c. One of our main tasks now is to mechanize agriculture.
d. Do you have anything to declare?
e. We have come to learn from you.

3) 不带to 的不定式:

a) 在“动词+ 宾语+不定式”结构中, 如果动词是表示感觉意义的see, hear, watch, smell, feel, notice等, 或是表示“致使”意义的 have, make, let等, 其后的不定式结构不带to.
John made her tell him everything.

这类结构转换为被动语态时, 后面的不带to 的不定式一般还原为带to 的不定式.
She was made to tell him everything.

b) 在 had better, had best, would rather, would sooner, would just as soon, might (just) as well, cannot but 等搭配之后, 动词不定式也不带to.
I’d rather not have eggs and bacon for breakfast.
They cannot but accept his term.

c) 在make do, make believe, let drop, let fall, let fly, let slip, let drive, let go of, let there be, hear say, hear tell, leave go of等固定搭配中, 用不带to的动词不定式.
John let fly a torrent¹ of abuse at me.
I’ve heard tell of him.

d) 在动词help(或help +宾语)之后可用不带to的不定式, 也可用带to的不定式.
Can I help (to) lift this heavy box?

e) 在介词except, but 之后, 如果其前有动词do的某种形式, 不定式一般不带to, 反之带to.
There is nothing to do except wait till it stops raining.
Smith will do anything but work on a farm.
There’s no choice but to wait till it stops raining.

f) 连词 rather than, sooner than 置于句首时, 其后的不定式不带to.
Rather than push the book back as he wanted to do, he forced himself to pick it up.

出现在句中其他位置时, 其后的不定式有时带to, 有时不带to.
He decided² to write rather than telephone.
The manager believes it is important to invest in new machinery³ rather than to increase wages.

g) 用作补语的动词不定式, 如果主语是由“all + 关系分句”，“thing +关系分句”，“what分句”或“thing +不定式结构”等构成，并带有do的某种形式，这时，作为主语补语的不定式可以省to，也可以不省.
What he will do is (to) spoil the whole thing.
All you do now is complete the form
The only thing I can do now is go on by myself.
The thing to do now is clear up this mess.
The least I can do is drive everybody else closer to the issue.

4) 不定式的其他用法

a) too…to 结构通常表示否定意义:
She was too young to understand all that.

enough…to结构则表示肯定意义:
She was not old enough to understand all that.

not too, but too, all too, only too等和不定式连用时,不定式一般不表示否定意义:
He’s only too pleased to help her.

so…as (to)这种结构也可用不定式作状语:
Be so kind as to drop in some time when you are free.

b) 如果要说明不定是表示的动作是谁做的, 可以在不定式前加一个for引起的短语:
It is not hard for one to do a bit of good.
It is a great honour for us to be present at this rally.

在以某些形容词(如 kind, good, nice, wise, unwise, clever, silly, wrong, right, foolish, stupid, careless, considerate, rude, naughty, impolite等)作表语时, 不定式前可加一个of引起的短语, 来说明不定式指的是谁的情况:
It’s kind of you to think so much of us.
(It is) Awfully⁴ good of you to come and meet us.
It’s very nice of you to be so considerate.
It’s unwise of them to turn down the proposal.