4．表示惊异、怀疑、不相信等态度，主要用于否定句和疑问句中。
How can you be so careless
你怎么这么粗心？
Where can could they have gone
他们会去了哪里？
He can'tcouldn't be over sixty.
他不可能超过60岁。
2． 表示允许、许可，常用在口语中。could比can语气上要客气。
--- Could I use your dictionary
我可以用一下你的字典吗？
---Yes, go ahead.
可以，用吧。(或Yes, you can. 但不能说Yes, you could.)
Could Can you tell me how to get to the zoo
劳驾，你能告诉我怎么去动物园吗？
He asked me whether he could take the book out of the reading room.
他问我可不可以把书带出阅览室。
3．表示推测，可用于肯定句(can罕见)、否定句和疑问句，但更常见于疑问句或否定句。在此种用法中can和could没有时态的区别，只是表示可能性的大小，can表示推测的可能性比could大。
Can he be ill at home
他会是生病在家吗？
Can the story be true
这个故事会是真的吗？
He cannot be at home.
他不可能在家。
You mustn't smoke while you are walking around in the woods. You could start a fire.
在林子里走时一定不要吸烟，那样可能会引起火灾。
1． 表示能力，可译为“能，会”。
I can swim. 我会游泳。
Emily can dance well and her mother could dance well when she was young.
艾米丽舞跳得很好，她妈妈年轻时舞跳得也很好。
The cinema can seat 1,000 people.
这电影院能容纳1，000人。
I could not read such an easy book when I was 7 years old.
我七岁时连如此简单的书我也看不懂。
4． 有时可以用于祈使句表示祝愿。
May you succeed.
祝你成功
May you be happy.
祝你快乐。
May that day come soon.
希望这一天早日到来。
B. can和could
3．用在目的状语从句中，构成谓语。
He wants to take a taxi so that he may get there in time.
他想坐出租车，这样他就能及时到达那里。
I arrived at the airport earlier in order that I might meet him.
我早早地就到了机场为了能接到他。
2．表示推测，可译为“可能，也许”。
Your math teacher maymight be in his office. 你们的数学老师可能在办公室里。(一般情况下，might表示的可能性很小)
The light isn't on. It maymight be broken.
那盏灯没有亮，它可能坏了。
There may might be some ink left in the bottle.
瓶子里也许还剩点儿墨水。
注意：
用may表示推测一般不用于疑问句，在疑问句中通常用can来代替。
Can he be at home
他可能在家吗？
--- Can it be true
这可能是真的吗？
--- It may be, or may not be.
可能是，也可能不是。
A. may和might
1．表示请求、许可，常译为“可以”。
在口语中可用can, could代替may，但在正式场合用may。表示允许时，也可用might代替，might不表示过去时，而是表示口气比较婉转。
You may take this seat if you like.
如果你喜欢可以坐这个位置。
MayCan Could Might I have a talk with you
我可以和你谈谈吗？
--- MayMight I come into the room to see my mother
我可以进房间看我母亲吗？
--- No, you mustn't. She needs to have a good rest.
不，你不能进。她需要好好休息。
提示：
May I ... 问句常见的肯定回答和否定回答。
肯定回答
Yes, please.
Certainly.
Yes, of course.
Sure.
Go ahead, please.
否定回答
No, you can't. (最常见)
No, you mustn't. (具有强烈禁止的意思)
Please don't. You'd better not.
I don't think you can.
I'm sorry it's not allowed.
4．情态动词+be done
This word can also be used as a verb.
这个词也可以用作动词。
Something must be done to stop pollution.
必须采取措施来制止污染。
The work ought to have been finished long ago.
这工作早就该完成了。
C. 情态动词第三人称单数一般现在时没有词形变化。
You ought to wear a raincoat.
你应该穿件雨衣。
She ought to wear a raincoat.
她应该穿件雨衣。
Plants must have oxygen in order to live.
为了存活植物必须有氧气。
A plant must have oxygen in order to live.
D. 情态动词的时态并不是区分时间的主要标志。在一些场合中，情态动词的现在式和过去式都可以表示现在时间、过去时间或将来时间。
I'm afraid it might rain tonight.
我看今晚可能要下雨。
Could I borrow your thermos
我可以借用你的暖水瓶吗？
It's a nice day today. We could go for a walk.
今天天气不错，我们可以出去散散步。
E. 情态动词是互相排斥的，一般不允许两个意义相近的情态动词连用。
[误]Soldiers must have to obey orders.
[正]Soldiers have to obey orders.
军人必须服从命令。
[正]Soldiers must obey orders.
[误]Can I be able to borrow two books at a time
[正]Can I borrow two books at a time
我能一次借两本书吗？
[正]Will I be able to borrow two books at a time
3．情态动词+have done
They might have visited the Great Wall.
他们可能参观过长城了。
He must have got up very early to catch the train.
他一定起得很早去赶火车了。
You ought to have come earlier.
你本该早一点儿来。
2．情态动词+be doing
She must be listening to pop music.
她肯定在听流行音乐。
You should be reviewing your lessons.
你应该在复习功课。
My mother may/might be cooking now.
我妈妈可能正在做饭。