can 与be able to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：
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.
Through his diligent work, the deaf-mute has been able to read and write in a sign language.
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和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.
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，即“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.
可能有同学会理解为“不能因为他为我们大家做了这些事情而过分表扬。”其实这样理解是错的, 因为这里的cannot be不是表“不能”，而是表“不可能是”的意思, 因此这句话的意思是“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”。
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.
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!
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.
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.
但是美国近来也出现了Being able to be hurt by things这种结构，美国专家解释为English is becoming looser. (英语变得越来越不严谨了)，中学生不宜模仿。