can / may / should / ought to / must / need等动词称为情态动词。情态动词有三大特征:一是无人称和数的变化,二是后面跟的动词不带to,三是后面 + not构成否定式,比较:
He neednt go there so early.
You do not need to be an expert to make this simple design.
在讲解情态动词完成式前,先将一般式中应注意的几个问题提一下,以引起必要的重视。
1.情态动词一般式中应注意的几个问题:
注意以下用法的习惯与区别:
He must be a doctor.
否定:He cant be a doctor.
又如:They cant be working now.
May I go now?
回答:Yes, please. / Please dont.
have to 与 must
1) 当强调外界原因时,2) 当与将来时结合时,用have to,
例如:
Since it is raining, we have to sty indoors.
You will have to hurry if you want to catch the 9:30 train.
3) 表示自然规律或必然结果,用must,
例如:
All men must die.
can 与be able to
can表示能力,但不一定做了或做成了;be able to 表示有能力并且做成了,相当于succeed in doing, 例如:
The patient is able to walk now.
The patient can walk now.
Shall I / we? 表示征求对方意见; + shall表示允诺或警告,例如:
Shall I fetch a doctor for you?
You shall be sorry for what you have done.
Will you.? 表示请求, 例如:
Don be late for the meeting next time, will you?
比较:I would remind you that smoking is forbidden here.
neednt 的肯定式为must,例如:
You neednt see him, but I must.
cannot but + 原形动词 , 例如:
I cannot but tell you the truth.
2.情态动词的完成式:
情态动词的一般式用于表达现在或将来的情况,而情态动词的完成式则用来表达已经发生的事情,例如:你不应该来得那么晚。这是对已经发生的事情表示自己的看法:You shouldnt have come so late.
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