can (could)
1）表示能力,could主要指过去时间.
Two eyes can see more than one. 两只眼比一只眼看得清.
Could the girl read before she went to school? 这女孩上学前能识字吗?
2）表示可能（理论上或是逻辑判断上）.
The temperature can fall to –60℃, that is 60℃ below freezing.
气温可降至—60℃,也就是零下60℃.
He can´t (couldn´t) have enough money for a new car. 他不可能有足够的钱买新车.
You mustn´t smoke while you´re walking around in the wood. You could start a fire.
在林子里走时勿吸烟,那样可能会引起火灾.
3）表示允许.
Can I have a look at your new pen? 我可以看一看你的新钢笔吗?
He asked whether he could take the book out of the reading—room.
他问他可不可以把书带出阅览室.
4）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.
Where can (could) they have gone to? 他们会去哪儿了呢?
He can´t (couldn´t) be over sixty. 他不可能超过六十岁.
How can you be so careless? 你怎么这么粗心?
5）比较委婉客气地提出问题或陈述看法.
Can (Could) you lend me a hand? 帮我一把好吗?
I´m afraid we couldn´t give you an answer today. 恐怕我们今天不能给你答复.
2. may (might)
1）表允许,might可以指过去时间,也可指现在时间,语气更委婉.
You may take whatever you like. 你喜欢什么就拿什么.
He told me that I might smoke in the room. 他告诉我可以在房间里抽烟.
May (Might) I ask for a photo of your baby? 我可以要一张你宝宝的照片吗?
在回答以may引起的问句时,多避免用这个词,而用其它方式,如Yes, please. / Certainly.
/ Please don´t ./ You´d better not. / No, you mustn´t.等,以免显得太严峻或不客气.
2）表可能（事实上）.可以指过去时间,也可以指现在时间,但语气更加不肯定.
He may be at home. 他可能在家.
She may not know about it. 她可能不知道这件事.
He was afraid they might not agree with him. 他担心他们可能不同意他的意见.
They might be having a meeting, but I´m not sure. 他们有可能在开会,不过我不肯定.
3. must
1）表示义务.意为“必须”（主观意志）.
We must do everything step by step. 我们一切都必须循序渐进地做.
You mustn´t talk to her like that. 你不可能那样对她说话.
--Must we hand in our exercise—books now? 我们现在就要交练习本吗?
--No, you needn´t. / No, you don´t have to. 不必.（这种情况下,一般不用mustn´t）
2）表示揣测.意为“想必、准是、一定”等,只用于肯定句.
He must be ill. He looks so pale. 他准是病了.他的脸色苍白.
She´s wearing a diamond necklace. She must have a lot of money. 她戴着钻石项链,一定很有钱.
4. shall
1）表征询意见,用于第一、第三人称疑问句.
Shall I get you some tea? 我给你点茶好吗?
Shall the boy wait outside? 让那男孩在外面等吗?
What shall we do this evening? 我们今晚做什么?
2）表说话人的意愿,有“命令、允诺、警告、决心”等意思,用于第二、第三人称陈述句.
You shall do as I say. 按我说的做.（命令）
You shall have my answer tomorrow. 你明天可以得到我的答复.（允诺）
He shall be sorry for it one day, I tell you. 有一天他会后悔的,我告诉你.（警告）
Nothing shall stop us from carrying out the plan. 什么也不能阻止我们执行这项计划.（决心）
5. will
1）表意愿,用于各种人称陈述句.
I will do anything for you. 我愿为你做任何事.
None is so blind as those who won´t see. 不愿看的人眼最瞎.
If you will read the book, I´ll lend it to you. 如果你愿意读这本书,我会把它借给你.
2）表请求,用于疑问句.
Will you close the window? It´s a bit cold. 请你把窗户关上好吗?有点冷.
Won´t you drink some more coffee? 再来一点咖啡好吗?
3）表示某种倾向或习惯性动作.
Fish will die out of water. 鱼离开水就不能活.
The door won´t open. 这门打不开.
The boy will sit there hour after hour looking at the traffic go by.
那男孩常常坐在那里好几个钟点,看着车辆行人通过.
6. should
1）表义务.意为“应该”（某件事宜于做）,用于各种人称.
You should be polite to your teachers. 你对老师应该有礼貌.
You shouldn´t waste any time. 你不应该浪费时间.
2）表推测,意为“想必一定、照说应该、估计”等.
The film should be very good as it is starring first—class actors.
这部新电影是一流演员主演的,估计拍得很好.
They should be home by now. 照说他们现在应当已经到家了.
7.would
1）表意愿.
They would not let him in because he was poorly dressed. 他们不让他进去因为他衣着破旧.
I said I would do anything for you. 我说过我愿意为你做任何事.
2）表委婉地提出请求、建议或看法.
Would you like another glass of beer? 再来杯啤酒好吗?
Would you mind cleaning the window? 请把窗户擦一下好吗?
They wouldn´t have anything against it. 他们不会有什么反对意见.
3）表过去反复发生的动作或过去的一种倾向.
Every time she was in trouble, she would go to him for help.
她每遇到麻烦都会向她求助.他告诉我盒子打不开了.
摘自：http://blog.sina.com.cn/s/blog_67292d4c0100ykek.html
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