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can，
could
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表示具有某种能力，指有能力做某事
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He could swim when he was five.
By the age of seven he could write beautifully.
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表示请求和允许，和may相近；口语中可用could表示委婉语气，但回答时仍用can
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—Could /Can I use your pen?
—Yes, of course you can.
Could I ask for your reaction to this policy？
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表示推测，常用于否定句和疑问句中
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Mike can't have found his car, for he came to work by bus this morning.
I think the work can be completed ahead of time.
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表示“有时也会”
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It can be quite windy there, especially in spring.
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cannot but do sth.表示“只好，不得不”
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Father cannot but agree with his daughter's opinion.
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can't help doing sth.表示“禁不住做某事”
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I can't help feeling sorry for the poor man.
The film is so impressive that we can't help crying.
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be able to
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be able to可以有多种时态，如：将来时(will be able to)，完成时(have /has /had been able to)
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I will be able to speak French in another few months.
I have been able to swim since I was 5.
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be able to侧重通过努力成功做成某事的能力
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He said he could swim across the river. (指本身具有这种能力)
At last he was able to swim across the river. (指通过努力做到了)
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may,
might
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表示允许、许可或征求允许，否定回答时要用mustn't; might比may语气委婉
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—Might /May I smoke in this room?
—No, you mustn't.
—May /Might I take this book out of the room?
—Yes, you can. (No, you mustn't.)
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表示可能性，might比may可能性更小
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I think the trip may give me time to calm down.
An active volcano may explode at any time.
Don't go too far afield or we might lose you.
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may /might as well do sth.“不妨做某事，还是做某事为好”
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Everything is ready. We might as well start now.
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will，
would
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表示意志、决心或愿望
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We will do our best to save the child.
I told her to stop crying, but she just wouldn't listen.
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用于第二人称，表示请求、建议等
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Will /Would you please post the letter for me?
Won't you take off your coat？
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will表示“总是，惯于”； would表示过去的习惯，意为“过去常常”
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When he was a child, he would often go skiing.
He would sit there for hours sometimes doing nothing at all.
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表示推测或猜想
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This will be the house you're looking for.
I think he would accept the invitation.
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shall
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用于第一、三人称的疑问句中，表示征求对方的意见或请示
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Shall I clean the desks one by one？
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用于第二、三人称的陈述句，表示说话者的命令、警告、许诺等语气
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He shall return that book tomorrow. (命令)
I shall think it over and let you know my idea. (许诺)
You shall fail if you don't work harder. (警告)
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用于第三人称，在条约、规定、法令等文件中表示义务或规定，意为“应该，必须”
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Candidates shall remain in their seats until all the papers have been collected.
Persons under 18 shall not be employed in night work.
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should,
ought to
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表示“应该”，常用于表示现在或将来的责任或义务；还可表示推测，指有一定依据的推测
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We should /ought to help each other and learn from each other in our work.
We ought to go and see Mary tomorrow, but I don't think we will.
It ought to be a fine day tomorrow morning.
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should还可表示惊讶，意为“竟会，居然”
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It's strange that he should be absent.
It seems unfair that this should happen to me.
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must,
have to
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must强调主观看法，表示“必须”，have to强调客观需要，意为“不得不”
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Yesterday morning I told my father that I must call on a friend.
You have to wear uniform on duty, don't you？
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must表示“偏要，非得”等含义，这种用法常用于疑问句、条件句中
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Must you waste so much time?
Must you make so much noise?
Why must you always interrupt me?
We were ready to leave, but the baby must catch cold.
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mustn't意为“不许，禁止”；若表示“没有必要，不必”时，用needn't或don't have to
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That car is my property; you mustn't use it without my permission.
You mustn't drive into a street with a “No Entry” sign.
You needn't talk to anyone if you don't want to.
You don't have to worry about me. I'm a good swimmer.
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must表示推测时，仅用于肯定句，但在反意疑问句中，其后面反问部分取决于must后面的动词形式
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You must be a newcomer, aren't you?
She must have been very young when she got married.
It must be your mother who you are thinking of.
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need
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表示“需要，必须”，常用于疑问句和否定句中； 肯定回答用must，否定回答用needn't
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You needn't have the bike repaired right away.
—Need I go back to Xiamen after I visited Fuzhou?
—Yes, you must. /No, you needn't.
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dare
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主要用于否定句、疑问句以及条件句中，有过去式形式dared
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—Dare she tell him what she thinks about him?
—She daren't say anything. He will only shout at her. “How dare you speak to me like that？” he will say.
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used to
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“过去常常”，含有今昔对比的意味，表示过去习惯的动作或状态现在已结束，而would表示过去反复发生的动作，但不含对比意味
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He used to get up at six every morning. (现在不这样了)
He would get up at six every morning and had a walk in the forest then. (现在是否还这样不得而知)
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had
better，
would
better
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had better /would better表示“最好……”(但一般都用had better)；would better还可表示“宁愿，宁可”
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You had better go to the factory and have a look.
I would better die before asking him for help.
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情态动
词＋have
done
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must have done用在肯定句中，表示对过去情况的肯定推测，语气坚定
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It must have rained last night, for the road is wet.
Tom must have taken your dictionary by mistake.
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can't have done表示对过去情况表示否定的推测，是must have done的否定形式
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Mr.Smith can't have gone to Beijing, for I saw him in the library just now.
Mary can't have stolen your money. She has gone home.
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could have done用于肯定句，表示过去本能完成某事但事实上并没有完成，意为“本来能够做”
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He could have passed the exam, but he was too careless.
He could have bought the car, but soon his company went bankrupt.
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may /might have done表示过去可能发生某事，意为“可能……，或许……”；might have done还可表示“本可能做某事”
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—What has happened to George?
—I don't know. He may have got lost.
She might have achieved greater progress, if you had given her more chances.
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should have done表示“过去本应该做某事而没有做”
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Tom, you are too lazy. The work should have been finished yesterday.
If I knew she was short of money, I should have lent her some.
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shouldn't have done sth.表示“过去本不该做某事而实际上做了”
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Look, Tom is crying. I shouldn't have been so harsh on him.
I don't think you have changed at all! Xiao Li shouldn't have failed to recognize you.
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needn't have done表示“过去本没有必要做某事而实际上做了”
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I needn't have bought so much wine—only five people came.
As you worked late yesterday, you needn't have come this morning.
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