Question

be able to和can的区别

Answer 1

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。

Answer 2

can能表猜测，be able to不能。

can只有两种时态, 而be able to 有多种时态。

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。

1）can、could 表示能力；可能 (过去时用could)。

只用于现在式和过去式(could).be able to可以用于各种时态。

They will be able to tell you the news soon.他很快就能告诉你消息了。

2）只用be able to 

a.位于助动词后；

b.情态动词后；

c.表示过去某时刻动作时；

d.用于句首表示条件；

e.表示成功地做了某事时，只能用was/were able to，不能用could。

He was able to flee Europe before the war broke out= He managed to flee Europe before the war broke out

can也可以用于表示“许可”，它可以表示现在许可或将来许可，过去许可用could表示，此时一律不能用be able to的某种形式替换。

Answer 3

can 与 be able to 用法比较
在一般现在时主动语态中表示“能”、“会”时,两者可互换,意义差别不大.如：
I can / am able to reach the apples.
但在以下几种情况中两者不能换用：
1、在词义上,can 除可以表示“能”、“会”外,还可以表示“许可”、“允许”或可能性,而 be able to 则不可.例句：
A :Can it be true?
B :No,it can't be true.
2、当表达将来的能力时,一般要用 will be able to； can 一般只有现在时和过去时两种时态形式.用于完成时等其它时态时,也用 be able to.例句：
1) I'll be able to speak English in another few months.
2) If I have a good sleep,I'll be able to work out the problem.
3) I've not been able to find my bike.
但是当人们现在决定将来是否有做某事的能力时,can 也是可以用的.例句：
I'm too busy today,but I can help you with your English tomorrow.
3、be able to 多表示“人”有能力或技巧做某事,一般不用于指物；而 can 表示人、物的“能”、“会”都可以.例句：
This kind of car can run very fast.It's new.
4、can 可以有被动语态形式,而 be able to 则不能用于被动语态.例句：
The mistake can't be corrected.
5、can 与 be able to 不能重叠使用.例句：
（误）He can be able to do it well.

Answer 4

从意思上来说：be able to 和can都有“能”的意思。

不同：be able to 可用于各种时态；can只用于一般现在时或一般过去时；

be able to 强调通过努力而获得的能力, 而can则强调自身已具有的能力

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Answer 5

前面的答案都没有问题，补充一点：
当can与be able to都表示能力的时候，前者带有主观意向，而后者更客观，强调的是能力。比如，I can do this because it is illegal.这里就带有主观意向，我能力可以做，但是我不能去做，be able to 则不能这么说。