被動式: 當不定式的邏輯上的主語是不定式所表示的動作的承受者時, 不定式一般要用被動形式.

It is an honour for me to be asked to speak here.

She hated to be flattered.

He wanted the letter to be typed at once.

This is bound to be found out.

There are a lot of things to be done.

She was too young to be assigned such work.

功用: 不定式可以作主語 (a), 賓語 (b), 表語 (c), 定語 (d) 或是狀語 (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.

不帶to 的不定式:

在“動詞+ 賓語+不定式”結構中, 如果動詞是表示感覺意義的see, hear, watch, smell, feel, notice等, 或是表示“致使”意義的 have, make, let等, 其后的不定式結構不帶to.

John made her tell him everything.

這類結構轉換為被動語態時, 后面的不帶to 的不定式一般還原為帶to 的不定式.

She was made to tell him everything.

在 had better, had best, would rather, would sooner, would just as soon, might (just) as well, cannot but 等搭配之后, 動詞不定式也不帶to.

I'd rather not have eggs and bacon for breakfast.

They cannot but accept his term.

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, 用不帶to的動詞不定式.

John let fly a torrent of abuse at me.

I've heard tell of him.

在動詞help(或help +賓語)之后可用不帶to的不定式, 也可用帶to的不定式.

Can I help (to) lift this heavy box?

在介詞except, but 之后, 如果其前有動詞do的某種形式, 不定式一般不帶to, 反之帶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.

連詞 rather than, sooner than 置于句首時, 其后的不定式不帶to.

Rather than push the book back as he wanted to do, he forced himself to pick it up.

出現在句中其他位置時, 其后的不定式有時帶to, 有時不帶to.

He decided to write rather than telephone.

The manager believes it is important to invest in new machinery rather than to increase wages.

用作補語的動詞不定式, 如果主語是由“all + 關系分句”，“thing +關系分句”，“what分句”或“thing +不定式結構”等構成，并帶有do的某種形式，這時，作為主語補語的不定式可以省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.