什麼是「不定詞」？What is 「infinitive」?

不定詞的一般形態是： to + root form
例如：

I go to swim.
I go to school.
* "to swim" 是不定詞；"to school" 不是。
不定詞可以有自己的賓語，亦可以被副詞修飾。
不定詞加上賓語或副詞所構成的短語，稱「不定詞短語」
Infinitive Phrase。

1. 作名詞
例如：
He wishes to be a doctor.
Do you know how to speak German?
To see is to believe.
His business is to sell newspapers.
※ 不定詞可作為 but, except, about 這三個介詞後面的名詞
並構成介詞短語。例如：
She wants nothing but to marry.
He knows nothing except to cry.
* 當不定詞可作為 but, except這兩個介詞後面的名詞並構成介詞短語時，
"to"可隱藏。即：
She wants nothing but marry.
He knows nothing except cry.
The child is about to cry.這小孩將要哭。
The old man is about to die.這老人將要死。

2. 作形容詞
例如：
Have you any rice to sell?
He has no friends to help him.
Has he no house to live in?
*「原裝」：He has no house in which to live.
「變裝」：He has no house which to live in.
「簡裝」：He has no house to live in.
The boy has no friends to play with.
He has raised his ability to listen, speak, read and write.
※ 在 "see", "watch", "notice", "observe", "hear", "feel',
"let", "make" 等動詞後面的不定詞短語，"to"要隱藏。例如：

Yesterday I saw him fly a kite.
Did you hear a bird sing in the tree this morning?
He let me drink it.他讓我喝。
He made me drink it.他逼我喝。

3. 作副詞
例如：
I shall go to America to learn engineering tomorrow.
Those days have gone never to turn.
I am sorry to strike you.
His income is too small to support his family.

* "enough" 放在所形容的詞後面。
* 不定詞短語所形容的副詞只有"too" 和 "enough"。
It is a good idea, to be sure!
* 不定詞短語 "to be sure" 形容整個句子 "to be sure"。
To tell you the truth, I love her very much.
§ 不定詞的時態與語態
不定詞本身可以有不同的時態和語態。現以 "do" 為例表列如下：
* 主動語態 被動語態
一般時態 to do to be done
完成時態 to have done to have been done
進行時態 to be doing /

※ 不定詞一般時態所表示的動作變化與謂語動詞所表示的動作變化
可以同時發生，或者在謂語動詞所表示的動作變化之後發生。
※ 不定詞完成時態所表示的動作變化在謂語動詞所表示的動作變化
之前發生。
1. I want to love her.我想去愛她。
2. I want to have loved her.我想已愛上了她。
※ 當謂語動詞所表示的動作變化發生時，若不定詞所表示的動作變化
正在進行，則不定詞要用進行時態。
They seem to be talking about something important.
她請求被派到那個部門。
I feel happy to have been asked to speak here.
我感到高興被邀請到這裡講話。