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.
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。
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。
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
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。
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。
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。
5.could可以用在虚拟条件句中。
if i could fly, i should be very happy.
如果我能飞， 我会很高兴。
if you had followed my advice, you could have finished it.
如果你当时听我的，你早就做完了。
if you had tried harder, you could have passed your exam.
如果你再努力些，你就能通过考试了。
6.can与be able to的区别
①can表示“能力”时，和be able to相当，许多场合都可以互相替换。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，表示你有能力时，只能用be able to。
【正】can you speak any foreign languages
你会说外语吗？
【正】are you able to speak any foreign languages
你会说外语吗？
【误】the fire spread the building quickly but everybody could escape.
【正】the fire spread the building quickly but everybody was able to escape.
大火迅速蔓延到整幢大楼，但大家都逃了出来。
【正】the fire spread the building quickly but everybody managed to escape.
大火迅速蔓延到整幢大楼，但大家都想法逃了出来。
②be able to 比can有更多形式。
no one could answer the question.
没人能回答这个问题。（这里could可用was able to 代替）
when he grows up, he will be able to support his family.
他长大后就能养家了。
frank is ill. he hasn't been able to go to school for one week.
弗兰克病了，已经一周没去上学了。
i'm sorry for not being able to help you in time.
对不起，不能及时帮你的忙。
③could经常和动词see, hear, smell, taste, feel, remember, understand等连用。
when we went into the house, we could smell something burning.
当我们走进屋子时，我们闻到什么东西烧焦了。（不用was able to）
she spoke in a very low voice, but i could understand what she said.
虽然她讲话的声音很低，但我还是明白了她说的话。
④在谈论说话时发生的动作，用can，不用be able to。
【误】look! i'm able to swim.
【正】look! i can swim.
看，我会游泳了！
c.must和have to
1.must表示有做某一动作的必要或义务，强调主观看法，可译为“必须，应该”。
everyone must obey the rule.
人人都必须遵守制度。
we must do everything step by step.
我们必须逐步做每件事。
teachers must be patient enough with their students.
老师必须对学生有足够的耐心。
2.have to 表示因客观需要促使主语不得不做某事。
you can't turn right here. you have to turn left.
你不能在这里右拐，你必须左拐。
my eyesight is very poor. i have to wear glasses for reading.
我的视力很差，看书时不得不戴眼镜。
you have to pass a test before you can get a driving licence.
你在取得驾照之前必须先通过考试。
提示：
在口语中，我们可以使用have got to来代替have to.
i've got to work on sunday.
我不得不星期天工作。
when has ann got to go
安必须什么时候走？
3.must 与have to的比较。
must 与have to在表示“必须”这个意思时意义相近，但在用法上有所区别。
①must含有说话者的强烈决意（表示主观的看法），have to 则表示外力环境或习惯使然（即表示客观的必要，作“不得不”解）。
i must study hard.
我必须努力学习。（发自内心的决定）
i have to study hard.
我不得不努力学习。（外界因素逼迫）
you must stay for supper.
你必须留下来吃晚饭。（because i want you to）
you have to stay for supper.
你得留下来吃晚饭。（because there is nowhere else to go）
②must只有一种形式，而have to可有多种时态。
they had to put off the sports meet due to the bad weather.
由于坏天气，他们不得不推迟运动会。
these last two days he has had to take a rest at home.
这两天他必须一直在家休息。
③must的否定式和have to的否定式的意思完全不同。
you must keep it a secret. you mustn't tell anyone.
你必须保密，你不可以告诉任何人。（mustn't表示“禁止”）
you don't have to tell me the secret. 你不必告诉我这个秘密。
（don't have to意思是“没有必要”= don't need to）
4.must用于表示推测，它的肯定程度比may, might, could大得多，一般只用于肯定句。
①must do (这里的do通常是状态动词或系动词) ，表示对现在状态的推测。
he must be very lazy for his desk is very untidy.
他肯定很懒，因为他的课桌很不整洁。
carol must get very bored with her job. she does the same thing every day.
卡罗尔肯定对她的工作厌烦了，她每天都做同样的事。
②must be doing表示对现在动作或状态的推测。
put on more clothes. you must be feeling cold with only a shirt on.
多穿点儿衣服，你只穿一件衬衣，肯定觉得冷。
③must have done表示对过去的动作或状态的推测。
the ground is wet. it must have rained last night.
地面是湿的，昨晚肯定下雨了。
i didn't hear the phone. i must have been asleep.
我没听见电话铃响，我一定是睡着了。
提示：
must表示推测只用于肯定句，否定句和疑问句要用can, could来代替must。
he's just had his lunch. he can't be hungry already.
他刚吃过午饭，不会饿的。
they have just arrived here. they can't know many people.
他们刚到这里，不可能认识很多人。
leslie walked past me without speaking. he can'tcouldn't have seen me.
莱斯利从我身边走过而没打招呼，他肯定没看见我。
5.must可用来表示根据逻辑推理必然要发生的事，可译为“必然会，总是会”。
we all must die.
我们都会死。
truth must be out.
真相必然会大白的。
winter must be followed by spring.
冬天到了，春天还会远吗？
6.must有时可用来表示“偏偏”的意思。
why must it rain today
为什么偏偏在今天下雨？
when i was taking a nap, a student must knock at the door.
正当我午睡时，偏偏一个学生在这个时候来敲门。
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