1.定义：具有名词、形容词及副词性质并具有时态、语态变化。
2.形式：（以do为例）
主动语态 被动语态
一般时 to do to be done
进行时 to be doing
完成时 to have done to have been done
完成进行时 to have been doing
3.用法：
（1）用作主语：
To speak good English is not easy.
or: It is not easy to speak good English.
（采用形式主语it以避免头重脚轻）
It took me an hour to do the work.
（2）用作宾语：
She decided to take the examination.
I hope to meet him soon.
（3）宾语补足语：
They expected us to help them.
Hewants his son to study hard.
金牌重点：
不定式作宾补，如谓语动词是感官动词，使役性动词（see, hear, notice, watch, make, have, let...），则不定式符号“to”须省略，但在被动语态中不能省。
I heard them sing in the classroom.
I made her clean the room.
The girl is heard to sing an English song.
