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【动 词(三)】
一、助动词
助动词本身没有词义，不能单独作谓语，只能和主要动词一起构成谓语动词，表示否定、疑问、时态、语态等语法形式，或用来加强语气。常用的助动词有:be, do, have, shall, will等。它们的具体用法如下：
(1)助动词be
A. 可用于构成进行时态
She is doing her homework now.
I asked him what he had been doing all afternoon.
B. 可用于构成被动语态
The baby was put in bed by his mother.
The invitation was received yesterday.
C. 可与动词不定式构成谓语
These books are not to be taken out of the room.
She is to arrive at six this morning.
(2)助动词have
A. 构成完成时态
I have not seen him for three years.
How long has your uncle taught in the village?
He had lived in Scotland for fifteen years before he came to England.
B. 和动词不定式构成谓语，表示因客观环境促使不得不做的事情
She had to go shopping yesterday.
(3)助动词do
A. 构成疑问句和否定句
Did anyone sharpen this knife?
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