can，could和be able to的区别
1、时态不同
从时态形式来看，can 只有现在式和过去式(could)，而 be able to 则可根据需要使用多种时态，还可以和一些情态动词连用，或用作动词不定式等非谓语形式。could是can 的过去式。
I haven't been able to sleep recently. 最近我睡不着觉。
He can swim. 他会游泳。
He couldn't be a bad man.他不大可能是坏人。
2、含义不同
用于现在时，can 泛指一般的能力，而 be able to 则主要指具体做某件事的能力。could不表示时态。
He can swim. 他会游泳。
I am able to express my idea in a clear way. 我能清楚地表达自己的想法。
Could I have the television on?你能打开电视吗？
3、表能力时不同
如果要表示"一时的能力"，即"在过去某时有某种能力，并顺利地完成某事"，则要用 was / were able to，而不能用 could。could 表示可能或有能力做。
Though he was sick, he was able to swim across the river. 虽然他病了，但他还是游过了河。
I could pass the examination. 我当时有能力通过考试。（有能力，但实际上不一定做得到）
can 与be able to在用法上既有相同的一方面, 又有不同的地方,这是应该引起注意的。当它们表示能力的时候，是同义的。如：
He can speak two foreign languages.
He is able to speak two foreign languages.
它们用法上的不同主要体现在以下几方面:
1. can只有两种时态, 即can 和could, 而be able to 有多种时态, 如was/were able to, will/shall be able to, have/has been able to等:
I’ll be able to drive the car in a week.
2．表示过去通过努力终于做成了某事要用be able to，而不能用can。如：
I was able to swim to the bank after the boat turned over.
can 和be able to 都可以表示能力.can泛指一般的能力,而且只有两种形式,即:can, could．be able to 则主要指具体做到了某件事的能力，其形式主要是靠be 发生变化，所以形式比can 多．可以说：I can swim. I am able to swim. 但是不能说：All the people could escape from the big fire in time. 只能说：All the people were able to escape from the fire in time.
3．can能表猜测，be able to不能。can的这种用法主要用于疑问句和否定句。
Somebody is knocking at the door. Who can it be?
It can’t be our teacher who is knocking at the door.
其肯定形式是must be。如:
It must be our teacher who is knocking at the door.
must have done something 表示对现在完成或过去动作的猜测，其否定形式是can’t have done。如:
“His parents must have gone abroad.” “No, I don’t think so. They can’t have gone, for I saw them only this morning.”
要特别注意它们的反意问句形式：
He must be a good teacher，mustn’t /isn’t he？
The students must have gone to bed, haven’t they?
They must have finished their lessons yesterday, didn’t they?
could既可以用肯定形式，又可以用疑问或否定形式。如果用could表示现在“可能”，比can语气更加委婉。如：
If you don’t spend enough time on your studies, you could fail in the final examination.
It could be our teacher who turned off the light last night.
与It must be our teacher. 相比,这句话表示的可能性更小, 因为它表示了两种可能性, 既 “可能是”也可能“不是”，而 must be 只表示了一种猜测。
could have done 这个句型有它的特殊含意，表示“本来可能做到而未做到的事”。如： Our class team could have won the game.
再看以下例句：
He cannot be over praised for what he has done for all of us.
他为我们大家做了这些事, 应该受到大大的表扬。 即 “再怎么表扬也不过分”。
4．can 可以表 “允许”, 与may可以互换, 此时不能用be able to 代替。如：
“Can / May I sit here？” “Yes, please.”
由于may not 有两个意思, 即 “不可能” 和 “不许可”, 因此使用时要特别注意。如：
He may not go home this weekend.
此句既可以表示 He will probably not go home this weekend.也可以表示 I don’t permit him to go home this weekend. 在口语中，表达这两种不同的含义是通过句重音来解决的。如：
He may `not go home this weekend. (I don’t permit him to go home this weekend.)
He `may not go home this weekend. (He will probably not go home this weekend.)
而在书面表达时不可能使用句重音的手段, 所以最好用can’t代替may not来表示 “不可能”：
He can’t go home this weekend.
5. can 是情态动词，只能用作谓语成分，be able to是普通动词短语，既可以作谓语，也可以作非谓语。如：
Being able to speak English fluently, he has no difficulty in communicating with the Americans at the party.
How I long to be able to communicate with the foreigners in fluent English!
6. be able to 后边接动词不定式表示一种实际情况时，其否定形式不是be not able to，而是cannot。如：
Were you able to catch the first bus yesterday morning?
No, I couldn’t.
7. 一般说来, be able to 后边的动词不定式没有被动语态。如:
I’m sure he is able to correct his mistakes in grammar.
不说: *I’m sure his mistakes are able to be corrected by him.
从时态形式来看，can 只有现在式和过去式(could)，而 be able to 则可根据需要使用多种时态，还可以和一些情态动词连用，或用作动词不定式等非谓语形式。could是can 的过去式。
I haven't been able to sleep recently. 最近我睡不着觉。
He can swim. 他会游泳。
He couldn't be a bad man.他不大可能是坏人。
2、含义不同
用于现在时，can 泛指一般的能力，而 be able to 则主要指具体做某件事的能力。could不表示时态。
He can swim. 他会游泳。
I am able to express my idea in a clear way. 我能清楚地表达自己的想法。
Could I have the television on?你能打开电视吗？
3、表能力时不同
如果要表示"一时的能力"，即"在过去某时有某种能力，并顺利地完成某事"，则要用 was / were able to，而不能用 could。could 表示可能或有能力做。
Though he was sick, he was able to swim across the river. 虽然他病了，但他还是游过了河。
I could pass the examination. 我当时有能力通过考试。（有能力，但实际上不一定做得到）
三个词都是能够，但是当我们讨论的事情与说话是同时进行的时候，不能用be able to
例： Watch me, Mum; I can stand on one leg. （一边说一边站，不能用I'm able to stand on one leg.）
动词被动式前不能用 be able to
例：Films can now easilly be streamed online. （不能用Films are now easily able to be streamed.）
想要表达知道如何完成的意思时不能用be able to
例：Can you cook? （不能用Are you able to cook?）
过去时中谈论一个成就而非普通能力时，比起could更倾向于用be able to
例： She swam strongly and was able to cross the river easily. （游过河是一个特定的成就，不能用could cross）
过去时的否定句中用could比be able to更合适
例：I tried to get up but I couldn't move.
和感官动词feel, hear, see, smell, taste,以及跟思想有关的动词believe, decide, remember, understand连用时，用could
例：I could remember the crash, but nothing after that.
在单词all和词组the only thing/place/time之后表示唯一能做用could
例：All we could see were his feet.
表示几乎没有发生，特别是和almost, hardly, just, nearly连用，用could
例：I could nearly touch the ceiling.
讨论理论上的可能性用could，讨论事实上很大概率会发生用can。
例：It could be expensive to keep a cat. （猜测如果有只猫，可能会烧钱）
It can be expensive to keep a cat. （事实上有人养猫就是很费钱）
如果是理论或现实中不可能实现的情况用can't，不能用couldn't
例： There can't be many people in the world who haven't watched television.
暗示将来的事情有很大可能性发生用can，暗示某事不太可能发生或者抱有怀疑态度用could
例：We can stay with Jake in Oslo. （会在一起）
We could stay with Jake in Oslo. （如果Jake在的话就可以待一起，说明人不在）
过去时里有权利在任何情况下做某事用could或 was/were allowed to，但是在过去某个特定情形下被允许做某事，用was/were allowed to，不能用could
例：Anyone was allowed to fish in the lake when the council owned it. （或者...could fish...）
Although he didn't have a ticket, Ned was allowed to come in. （不能用...could come in）
过去时的否定句中，可以用couldn't或wasn't/weren't allowed to 表示在任何情况下都不被允许
例：I couldn't/wasn't allowed to open the present until my birthday.
在这个时候 can=could=be able to 大致上，could 比 can 稍微有点距离感，像是说话能有点退路。而通常 具体某件事情的时候 不会说 can 或者 could.
icandrive.我可以开车。icoulddobetter.我有能力做的更好，但是比can的态度稍微没那么坚决。i am able to buy a car. 这个时候说具体的某一件事情的时候。我们说beableto.
