新东方戚乐：情态动词系列辨析
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情态动词
PART I
1. can, could
(1)表示能力或客观可能性、请求或允许：
Man cannot live without air or water.
(2)表示惊异、怀疑、不相信：
How can you be so careless!
He couldn’t be over sixty.
(3)表示对过去否定或疑问的猜测：
Jack cannot (couldn’t) have been to China, has he?
Can (Could) he have heard the news?
(4)表示虚拟语气，用于肯定句，could have done,“过去能够干
某事而没干”。
I could have lent him money, but he didn’t ask me.
Given more time, I could have done it even better.
(5)表示“有时会”：
Training alone can be dangerous.
2. may, might
(1)表示推测“可能”：
That may or may not be true.
(2)对过去可能性的推测，may (might可能性更小)have done：
She may not have seen the film.
She might have had an accident.
(3)表示虚拟语气“有可能干而没干”：
She might have given you more help, but she didn’t.
(4)表示祝愿：
May you succeed in passing the exam!
3.must，have to
(1)“必须”，“必要”：
Soldiers must obey orders.
You mustn’t talk like that.
(2)肯定的猜测(不能用于否定或疑问)：“一定”：
He must be seventy now.
He must be doing his homework upstairs.
I think you must have made a mistake, didn’t you?
He must have been doing his homework then.
(3)(表示与说话人愿望相反及不耐烦)偏要
Just as I was hurrying to class, Mary must come, telling a
long story.
4. need
(1)“需要”，主要用于否定、疑问句。
You needn’t do it at once.
(2)needn’t have done,“过去不需要干而干了”：
You had enough time left. You needn’t have hurried.
(3)特殊句型，“需要干某事”：
The tree needs (wants, requires) watering (to be watered).
5.dare，dared
(1)主要用于疑问、否定和条件句中：
How dared they do such a thing?
(2)可作实意动词：dare (to) do:
He didn’t dare (to) go there.
6. shall, should
(1)shall用于第一、第三人称疑问句中，征求对方意见：
Shall I turn on the radio?
(2)shall用于第二、第三人称陈述句中，表示允诺、警告、命
令、决心等。
You shall fail if you don’t take his advice.
Nothing shall stop us from carrying out this plan.
(3)should/ought to,“应该”：
You should work hard at your lessons.
(4)should(ought to) have done,“过去本来应该干某事而没干”;
should not have done“过去本来不应该干某事而干了”,表示
委婉的批评：
You should have come earlier.
You shouldn’t have told her about it.
You ought to have told me about that yesterday.
7. will, would
(1)用于疑问句，表示请求、邀请：
Will/Would you pass me the book?
(2)“愿意”：
If you will wait a little while, I’ll call the manager.
(3)“总是”：
He will often sit up all night.
He would drop in on me on Sundays.
(4)用于否定句，表拒绝，尤指物：
We pushed the bus, but it wouldn’t move a bit.
8.used to “过去曾经……”，暗示现在不再如此：
You used to smoke, usedn’t you (didn’t you)?
PART II
l 比较can /could 和be able to表示“能力”的含义
can / could 表示能力，可能 (过去时用could)，只用于现在式和过去式(could);
而be able to可以用于各种时态。
l 比较can /could 和may表示“可能性、委婉的请求和许可”的含义
1、 may 与can 在表示可能性的时候，前者的可能性小于后者。may的委婉程度要比can深。May用于正式语体，can用于非正式，常见于“会话”中。
2、 may用于肯定的可能性推测，而表示不可能时则用can not。表示征求许可时用may，其肯定回答用can，否定回答用can not。
l 比较must 和 have to表示“必须”的含义
1、 must 与have to 都是强调要去做一件事的必要性，但究其原因，must，“必须”是强调自身的原因，表示说话人主观上的看法;而have to ，“必须”则强调外界的原因，表示客观需要，可以理解为“不得不”。
2、 have to有人称、数、时态的变化，而must却没有;在否定结构中要注意“语气”，don't have to 表示“不必”;mustn't表示“禁止”或“一定不要”。
例如：You don't have to tell him about it. Vs You mustn't tell him about it.
l 比较need “情态动词”和“实义动词”的用法
1、 need作为情态动词，一般不用于肯定句，只能用于疑问句，否定句。例如：
1) Here comes the bus. So I needn’t take a taxi. (needn’t =need not)
2) Need I type this paper again? Yes, you must. / No, you needn't.
3) Should I tell Mike about it? No, you needn’t. I’ve told him already.
2、 need 作实义动词时，表示“需要，要求”，后面接名词或跟带to的动词不定式，(need + n. / to do sth. )。在构成否定句和疑问句时，要借助于助动词do。此时的need，既可以用于肯定句，也可以用于否定句和疑问句。
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