1) can / could
e.g. -- Can I go now?-- Yes, you can.
Can you pass me the books?你能给我递一下书吗 ?
Could you help me, please?请问,你能帮助我吗?
“-- Could I come to see you tomorrow?
-- Yes, you can.” （否定答语可用No, I'm afraid not.）
could语气委婉，主要用于疑问句，答语应用can.
2) may / might
might比may的语气更委婉，否定回答时(口语中常用) no , you can't. or, yes, please
e.g. “-- May I use your dictionary?-- Yes, please.”
“-- Might I use your pen?-- No, you can't. ”
You may drive the car.
You may take the book home.
“-- May I come in?-- Yes, please.”
“---Where can he be?” “—He may / might be in the office.”
(用May I ... 比较正式、客气 。在口语中，常用Can I ... 征询对方意见 。)
3) mustn't表示“不可以”、“禁止”、“阻止”之意(具有强烈禁止的意思)如：
e.g.“May I take this magazine out?” “No, you mustn't.”
“我可以把这本杂志借走吗?”“不, 不行 。
4. obligation义务性的必须: “must / have to / ought to / should”
e.g. The children must be back by 4 o'clock.
You must remember to write home.
We mustn't waste our time.
You ought to have asked him (but you didn't).
这时，ought to和should可以互相换用 。
Ought you*** oke so much?
You oughtn't*** oke so much.
1）must最强，ought语气略强、正式，should较常用 。
2）must 表示说话人的主观思想, have to 表示客观需要 。
You must do it now. 你必需现在就干 。(说话人认为必须现在干)
You must come in time.
I have to go now.我得走了 。(不想走，但客观条件必须走 。)
3）回答must引出的问句时，如果是否定的回答，不能用mustn't，而要用needn't或don't have to 。
e.g. — Must we hand in our exercise books today?
— Yes, you must. (No, you don’t have to.)
4）在疑问句中，通常用should
e.g. Should I open the window?
5) 表示应该做到而实际上没有做到，并包含一种埋怨、责备的口气 。
e.g. She should have finished it.
He should have helped her, but he never could.
You should have started earlier.
Why must you always bother me?
5. special feelings表示惊异、怀疑、不快、失望等态度和感情：“should / can / could”
(to denote emotional feeling of displeasure, surprise, wonder, disappointment, etc.)
e.g. It’s a pity (that) they should lose the game so soon. (disappointment)
How should he speak such rude words to his mother? (surprise)
It is unbelievable that he should be working so hard.
How could I know? 我怎么会知道呢？
Why should you be so late today? 你几天怎么来得这么晚？
— Where is Betty living? — 贝蒂住在哪里？
— How should I know? — 我怎么会知道呢？
I don't know why you should think that I did it.
我真不知道你凭什么认为这件事是 *** 的 。
6. prediction 预见: “will”:
e.g. This will be the book you want.
He will have arrived by now.
He will be a different person when he lives in England.
7. intention / resolution / will / warning: 说话人的意愿 / 意图 / 决心 / 警告:
“shall, will”
(Note: used for the 2nd and 3rd persons包括用于第二和第三人称作主语)
e.g. You shall stay with us as long as you like.
= I’m willing to let you stay with us as long as you like.
He shall be praised if he works hard this term.
= I’ll see that he is praised if he works hard this term.
I will never do that again.
They shall not pass. == We won’t let them pass. （warning）
You shall fail if you don't work harder.
He shall be punished.
He shan’t come here == I won’t let him come here.
You shall obey my orders. == I insist that you obey my order.
He shall leave the country at once. == I insist that he leave the country at once.
8. ask for advice征求意见： “shall”
e.g. What shall we do this evening?
Shall we begin our lesson?
Let’s go together, shall we?
9. politeness / euphemi ***/ modesty: 礼貌 / 委婉 / 谦虚: “should, would”
e.g. Would you pass me the book?
Would you kindly show me the way to the station? 你可以指点我去车站的路吗?
If you would kindly wait here a minute, I’ll ring the director’s office.
I should think it would be better to try it again. 我倒是认为更好再试一试 。
You are mistaken, I should say. 依我看，你是搞错了 。
I should advise you not to do that. 我倒是劝你别这样做 。
This is something I should have liked to ask you. 这是我本来想问你的 。
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