情态动词的使用说明

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一、概述
情态动词是一种本身有一定的词义,表示说话人的情绪、态度或语气的动词,但不能单独作谓语, 只能和其他动词原形构成谓语。例如：
He must be the man I am looking for.
What can I do for you?

二、情态动词的基本用法
1. can (could) 表能力、允许、(从理论上或逻辑上判断）可能性以及表惊异、怀疑、不相信等态度（主要用于否定句、疑问句或感叹句中）。could指过去或表示语气委婉。例如：
The boy can speak three languages.

Can I borrow the book from the library?
I could swim when I was eight.
2. may (might) 表允许、可能性、祝愿等。might可以指过去，也可指现在，语气更委婉。例如：
May (Might) I ask for a photo of your baby?
She may be still waiting for us.
May you have a happy holiday!
3. will (would) 表意愿(用于各种人称的陈述句)、请求(用于疑问句)、某种倾向或习惯性动作等。would指过去或表示语气委婉。例如：
Will you come this way, please?
Would you please close the window?
Fish will die without water.
Every time she was in trouble, she would go to him for help.
4. must 表命令、推测（现在、过去或将来的猜测）、偏偏等含义。例如：
We must do everything step by step.
He must be ill. He looks so pale.
It can’t help. He must go with me.
5. shall 用于第一、三人称疑问句中表示请求或征求对方意见; 用于第二、三人称陈述句中表示允诺、命令、警告、威胁、决心等。例如：
Shall I get you a cup of coffee?
You shall have my answer tomorrow.
He shall be sorry for it one day, I tell you.
Nothing shall stop us from carrying out the plan.
6. should / ought to 表义务（因责任、义务等该做）、推测、建议等。ought to 的口气比should稍重。例如：
You should be polite to your teachers.
You are his father so you ought to take care of him.
I have bought three boxes. That should / ought to be enough.
三、有些情态动词可以与进行时连用，表示某些事情可能正在发生。有些情态动词可以与现在完成时连用，推测过去（可能）发生了某事。例如：
—What can they be doing now?
—They must be waiting for us. Let’s hurry up.
You must have gone to bed late last night. Your eyes are red.
四、部分情态动词的回答方式

疑问式	肯定回答	否定回答
Could I…?	Yes, you can. Yes, go ahead.	No, you can’t.
Must I…?	Yes, you must.	No, you needn’t / don’t have to.
May I …?	Yes, of course. Yes, please. Yes, go ahead.	No, you mustn’t. No, you can’t.
Will you ...? Would you…?	Certainly. / Sure. / All right.	I’m sorry. I can’t. No, thank you. No, I won’t.

五、辨析
1. can (could) 与be able to
can表示“能力”时，和be able to相当，许多场合都可以互换。但当叙述过去经过一番努力才能完成的事情或前面有特殊说明时，只能用be able to，且be able to可以用于各种时态。例如：
I can / am able to stand on my head.
I talked with her for a long time, and eventually I was able to make her believe me.
I’m sorry I haven’t been able to answer your letter in time.
2. must与have to
must与have to都含有“必须”之意，must含有说话者的强烈决心 (表示主观的看法)，have to则表示外部因素或习惯使然 (即表示客观的必要，作“不得不”解)，且have to 有更多的时态形式。例如：
He said that he must work hard. （主观）
My brother was very ill, so I had to call the doctor in the middle of the night. (客观)
We’ll have to help them as much as we can.
3. would 与used to
would表示过去的习惯性动作时，只表示过去的情况，与现在无关，往往要带有一个特定的时间状语；used to可表示过去的习惯动作或状态，强调“现在已无此习惯”。例如：
Whenever we were in the country, we would play hide-and-seek in the fields.
We used to play hide-and-seek in the fields.
4. mustn’t与needn’t
mustn’t表示“禁止”，用于告诉别人不要做某事，而needn’t则表示没有必要做某事(= don’t have to)。例如：
You must keep it a secret. You mustn’t tell anyone.
You needn’t light a match; I can see well enough.