助动词有哪些
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
- 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.
- 可用于构成被动语态
The baby was put in bed by his mother.
The invitation was received yesterday.
- 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
- 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.
- 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
- 构成疑问句和否定句
Did anyone sharpen this knife?
I don’t think you are right.
B．用来加强语气
I do want to have a talk with you
Do come and see me.
C．用来代替动词词组
Have you finished your work? Yes, I did yesterday.
He plays basketball well. So does his brother.
（4）助动词shall
构成将来时态，单纯表示未来情况
I shall not be back tonight.
We shall pay a visit to the Banpo ruins next month.
（5）助动词will
构成将来时，用于第一、二、三人称。如：
They will move to a new house.
She will not eat any solid food.
注意：
（1）have to 与must的区别
must是情态动词，意思是“必须”、“应该”、“一定要”，后接动词原形。Must的否定形式为mustn’t.构成疑问句时，肯定回答一般用must。否定回答用needn’t或don’t have to 表示“不必”。如：
Must I do it now ?
Yes, you must No, you needn’t.
have to 表示“不得不，必须”，可用于多种时态。
I missed the train, so I had to take a taxi.
I’m sorry I won’t go with you, I’ll have to go to the bank.
We don’t have to install this new television set.
（2）used to 表示过去常常，而现在不再有的习惯。其否定形式为: used not to, usedn’t (usen’t) to , didn’t use to.
I used to go to work by bus. Now I go by car.
I used to collect stamps when I was a boy.
Used they to have a lot more free time ?