情态动词

    在英语中，情态动词属于助动词。除了作简略回答外，它们一般不能单独使用，它们必须与动词原形一起使用构成句子的谓语。不同的情态动词会有不同的意义和用法。有时，同一个情态动词在不同的语境中也会产生不同的含义。下面我们将讨论这些情态动词的用法。

情态动词的推测性用法和非推测性用法

    情态动词一般有两种用法：推测性用法(epistemic use) 和非推测性用法 (non-epistemic use) 。推测性用法是指做出推测,预测事物发生的可能性; 非推测性用法则指情态动词本身的基本含义，如can表示能力，may表示许可，must和should表示必须和义务，need表示需要等。例如：

I can swim. (非推测性用法)

This can be true. (推测性用法)

You may go now. (非推测性用法)

He may be ill now. (推测性用法)

You must finish it before lunch time. (非推测性用法)

He must be at home for he never goes out at dinner time. (推测性用法)

我们把英语中的9大情态动词按其推测性和非推测性用法列表如下:

情态动词非推测性用法的说明

1.  may和might表示允许

may可以与不同的人称搭配，表示“许可、允许”。例如：

May I know your name?

You may be the first to leave if you are in a hurry. (我允许你第一个离开。)

You may keep the book for two months. (我允许你借。)

In certain circumstances a police officer may ask a driver to take a breath test.

may用在疑问句中，经常表示“允许、许可”的意思。例如：

May I know your name?

May I come in?

might可以用来表示过去时态，也可以表示委婉语气。例如:

She said that he might take her dictionary. (may的过去式)

Might I have a word with you? (委婉用法)

I wonder if I might have more salad. (委婉用法)

但在回答中，我们一般用may加强肯定的语气，而不用might.例如：

Yes, of course you may.

may not可用来表示“禁止”或“不许”，因语气较强，所以不用might not.例如：

Students may not stay out of the dormitory after midnight without written permission.

must not也可以表示“禁止”，而且语气比may not更强，强调上级对下级的不允许。

另外，may/might as well也是一个常用的词组，后接动词原形，意思是“不妨，最好”

相当于had better.例如：

It's very late. We might as well go home.

2. can和could

1)  表示能力

can,could和be able to都可以表示能力。

但若要表示在过去某个具体场合下能够做某事的能力，我们用was/were able to.  例如：

— Thank you for your help.

— I‘m glad I was able to help you. (在这种情况下能够帮你，而且已经帮了。)

所以was/were able to不仅表示过去的某个具体场合下的能力，还强调动作已经成功完成，相当于manage to do或succeed in doing.例如：

We were able to save him. (把他救活了。)

He was able to explain what had happened. (他能解释，而且也解释了。)

He could explain what had happened. (他能做出解释，但可能没有说。)

但这种区别只局限在陈述句，在否定句中，be able to和could可以互换。

有时，could表示的并不是can的过去式，而是一种委婉的语气。例如：

Could you run the business yourself?

I could give you a chance.

Could you please give me your reply as soon as possible?

could have done有虚拟的含义，即“本来能够做，但事实上却没有做”。例如：

You could have told me earlier.

I could have helped her.

I could have passed the exam if I had worked harder.

2)  表示许可

can也可以用来表示“允许”，例如：

Can I smoke here?

You can use my dictionary now.

The policeman says we can't park our car here.

could可以使句子更显礼貌，但在肯定回答中，我们一般用can代替could,加强肯定的语气。

即：Yes, of course you can.

3)  其他用法

can not与help, bear, stand等动词连用，表示“禁不住，受不了”。

例如：

If one had talent, one can't help showing it.

I can't stand waiting any longer.

但是can’t help but 后面接动词原形，表示“只能”。

I can not help but ask you: “do you have loved me?”
I can’t help but wait.

3. must表示必须

must的这种用法，体现了说话者的权威性，所以must通常用于上级对下级、长辈对小辈。

例如:

You must be back by ten o'clock.

    Teacher: You must use a dictionary. I’m tired of your spelling mistakes.

［注意］ must和have to的区别： must表示出于说话者本人的主观愿望，必须去做某事；而have to表示受外部条件的影响，不得不做某事。

例如：

I must leave now. (我自己想离开，主观愿望。)

I have to leave now. (也许我还想坐一会儿，但我有要事，不得不走了。)

He must say it in English. (除了英语，他可能还懂其他语言，但我的主观愿望是他必须说英语。)

He has to say it in English. (客观条件是他只懂英语，所以不得不用英语说。)

另外，由于must没有将来时态，所以我们通常用will／shall have to来表示；由于must没有过去时态，所以我们通常用had to来表示。

例如：

We will have to do it again.

I had to leave at 6:30 yesterday.

但有时，我们也可以在表示过去的上下文中使用must.

例如：

I told him that he must mind his own business.

must有两种否定形式： mustn't和needn't／don't have to.

mustn't表示 "不可以“，needn't／don't have to表示“不必”，

例如：

You mustn't talk like that. (不可以 = You are not allowed to talk like that.)

— Must you leave so soon?

— No, I needn't.

4. should和ought to

should和ought to通常可以交互使用，意义没有多大的区别，表示“应该”。should和ought to的这种用法通常表示“建议、敦促或怂恿”。

［比较］ must, have to和should／ought to所表示的不同语气：

     钢琴老师对某学生说：

    You must practice at least an hour a day. (must 显示了老师的权威。)

    这个学生然后对他的朋友说：

    I have to practice an hour a day! (have to表示我本身不想练，但迫于老师的压力，不得不练。)

他的朋友会说：

    You ought to／should practice for more than an hour. (表示不是来自外界的权威或压力，而是来自平辈同学或朋友的建议：如要做个好学生，弹好钢琴。)

另外，should have done／ought to have done表示“本该发生的事没有发生；

例如：

    You should have told me earlier.

    You ought to have been more careful.

5. will和would

will和would作情态动词使用，主要有以下的用法：

1)  表示意愿

will和would表示的意愿可强可弱，意愿弱时表示“愿意做某事”(willingness) ;意愿强时表示“坚持要做某事”(insistence) .例如：

    Will you go with us?(愿意)

    Will you pass me the salt? (愿意)

    Let us have a talk, will you? (愿意)

    I will marry her although my parents are strongly against our marriage.(表示坚持)

would可以表示“过去的愿意”， 一般用于间接引语中，例如:He said he would come.

在其他情况下，would不表示过去，而表示"委婉的语气“，例如：

    Would you please make a copy for me?

但在回答中，为了加强肯定的语气，我们用will.例如：

     — Would you let me hear from you soon?

     — Yes, I will.

2)  表示能力、趋势或必然性

will和would的这种用法一般以东西作为主语，表示“其内在的性能或特征性倾向”。 例如：

     Oil will float on water.

     Gold won't dissolve in acid.

否定形式won‘t还可以表示“某事没有达到我们期望的效果”。 例如：

    The car won't start.

    The door won't open.

   ［注意］这里用主动语态。

3)  表示习惯性的动作。例如:

Every morning he will sit in the office doing nothing.

He will often say something and then forget it.

在这种用法中，would是will的过去式。

例如：

He would never let anybody know what he was doing.

   In those days he would work day and night, with no time to care for his children.

6. shall

shall通常有以下用法：

1)  表示“说话者的决心或承诺”。

例如：

     I shall never give up.

     The invaders shall be wiped out.

     You shall get your salary tomorrow. (I promise...)

     He shall get his share. (I promise...)

     You shall stay with us as long as you like. (I promise...)

2)  表示“命令”，语气很强。

例如：

     You shall go at once.

     You shall do as I say.

     You shall never tell lies.

此种语气因太专横，所以一般不用，但在法律文件和合同条款中，我们通常用shall 表示权利和义务。

例如：

Party A shall advise Party B promptly concerning any information that comes to Party A's attention...

    Party A shall, within 14 days of receipt of a written request by Party B, deliver to Party B a written report on Party A's activities ...

    Party A shall be an independent contractor in all its operations.

3)  疑问句中用来征求意见。

例如：

     Shall I pick you up at six? (Do you want me to pick you up at six?)

Shall we go together?

Shall he go on an errand? (Do you want him to go on an errand?)

Let's have a rest, shall we?

7.need

need可以作行为动词，也可以作情态动词。need作行为动词时，有时态的变化，后面跟动词不定式或名词，其否定式和疑问式要用助动词；need作情态动词时，后面只跟动词原形，其否定式和疑问式不用助动词，而是直接在need后面加上not构成否定式，疑问句直接把 need提前。    

例如：

   You needn't come. (情态动词)

   I wonder if we need take sleeping bags. (情态动词)

   I need your help. (行为动词)

   I need to have a look. (行为动词)

   — Need we work today? (情态动词)

   — Yes, we must. (No, we needn't.)

need作情态动词，表示“需要”，常用在否定句、疑问句或if引导的从句中，在回答need引导的问句中，肯定回答用must加强语气，表示“必须”； 否定回答用needn‘t, 表示“不必”， 相当于not have to.

Needn‘t have done表示“没有做某事的必要，该事本不必做，但却做了”； don’t need to只是表示“不必做某事，而且该事也确实没有做”。

例如： 

I didn't need to go to the station. (没有去车站)  I needn't have gone to the station. (去了车站 )

8. dare

dare可以作行为动词，也可以作情态动词。dare作行为动词时，有时态的变化，后面跟动词不定式，其否定式和疑问式要用助动词；dare作情态动词时，后面只跟动词原形，其否定式和疑问式不用助动词，而是直接在dare后面加上not构成否定式，疑问句直接把dare提前。例如：

    I wonder how she dared to say that. (行为动词)

    We wouldn't dare to play with you. (行为动词)

    Does she dare to go there alone? (行为动词)

    Dare you jump over the ditch? (情态动词)

    She dare not go out at night. (情态动词)

    I dared not tell him the bad news. (情态动词 )

9. had better

had better的意思是“最好”，表示建议，相当于It‘s better for you to....had better后跟省略了to的动词不定式，即had better do; 其否定式为had better not do, 表示“最好不”。

例如：

     We had better start now.

     You had better not take the risk.

情态动词的推测性用法

用于推测性含义时，might可能性最小，must可能性最大。

1. may和might表示可能性

表示“可能性比较大”时，我们用may；如果可能性比较小，我们用might。

例如：

     He might tell his friends. (可能性小)

     He might be waiting at the gate.(可能性小)

     He may be at home. (可能性大)

     Tom may lend you the bike. (可能性大 = It is very likely that...)

     Tom might lend you the bike.(可能性小 = I think it is unlikely.)     

might也可以表示“过去的可能性”。

例如：If you invited him，he might come.

若要表示“不可能”，我们用can’t或couldn’t。

例如：

He can’t be the manager. He is only a junior clerk.

在问句中，may只能用来表示“允许”，不能表示可能性；否则，我们用can, do you think, is it likely that, is it possible that等结构来表示可能。

例如：

— May I come in? (表示许可)

       — Yes, come in please.

       — Where can he be? (表示可能性)

       — He may be in the office.

may可与have done连用，构成may have done,表示“某一事件在过去发生的可能性”。

例如：

— Why hasn’t she come?

      — She may have missed the train.

      --He isn’t back yet. He may have had an accident.

另外，might have done还有虚拟的含义，表示“与事实相反”。

例如：

You were stupid to try climbing up there. You might have killed yourself.

2.can和could表示可能性

can和could可以表示“事物实际发生的可能性”。如果逻辑判断是现在或说话时刻作出的，用can\could用来判断可能性，不确定性和委婉的语气要更强一些。

例如：

Can／Could the news be true?

     If you don’t have a guide, you could lose your way.

另外，can的这种用法多出现在否定句中，表示“不可能”。

例如：

It can’t／couldn’t be true.

The moon can’t always be at the full.

She can’t be serious.

can not have done表示“过去的时间发生的可能性很小”。

例如：The door can’t have been open for I locked it myself.

3. must表示极大的可能性

 must的此种用法，指的是逻辑上较大的可能性，表示“一定\肯定”,比will和should语气更强。

例如：

You must be mad to do that. (干那事你一定是疯了。)

must表示“推测”，通常只用于肯定形式，其否定形式为can’t。

例如:

 You must have difficulty getting the tickets.

     You can’t have any difficulty getting the tickets.

对过去动作的推测通常用must have done来表示，否定形式是can’t have done。

He must have taken sleeping pills last night.

You can’t have left your handbag in the theatre.

I don’t think he can have heard you. Call again.

1) — Can I have some sweets? I am hungry.

— You can’t be hungry. You’ve just had dinner.

2)  — A man answered the telephone. I suppose it was her husband.

— It can’t have been her husband. He has been dead for ages.

must表示猜测的可能性比may的可能性大许多。比较：

1)  钥匙圈上有三把钥匙，其中一把是开房门的。当我们拿起其中一把时，我们说： This may／might be the key. (当试了两把钥匙门都没有开后，我们拿起第三把钥匙。) 这时我们说: This must be the key. (因为这是最后一把钥匙了。)

2)  看下面的对话：

— I wonder why Tom hasn’t answered my letter.

— He might be ill. (他可能病了，但也有其他的可能，如他太忙了，等等。 )

4. should和ought to表示极大可能性

should 和 ought to 的这种用法指通过逻辑推断得出的极大可能，可以理解为“应该是” (= It is extremely likely that...) .

例如： That should not be a difficult problem for Kate. (不应该是个难题。)

            It should be ten o’clock now.

5. will和would表示预言

在这种用法中，will／would表示“情况肯定会是这样”。但would的可能性比will小。

例如：

    1)  — Where is Tom?

         — He will be in the library.

    2)  — There is someone coming.

         — It will be Mary.

比较 will 和 should 在推测性用法中语气的差异：

      Tom should know the address. (我想Tom应该知道 = I expect Tom knows it.)

      Tom will know the address. (我肯定Tom会知道的 = I am sure Tom knows it.)

从以上的例子中不难看出，will用来表示“猜测”时，语气比should更加肯定。另外，should表示“推测”不能用于贬义的(或令人感到不悦的) 语境中。

例如：

      Let’s not go shopping now. The shops will be very crowded. (这里不能用should)

      Let’s go shopping now. The shops will/should be fairly empty. (这里也可以用ought to)