1 be able to和can表有"能","能够"之意,一般可以互换.注:"be able to"中含有"be"动词,在句中运用时,be动词有明显的人称或数以及时态的变化,考虑用is,am,are,were或was,而can为情态动词,无人称及数的变化,"can"的过去式为"could".can无将来时.
2 "be able to"用在将来时句中时可以与"will"一起连用,但can不能与will一起用,"can"与"be able to"不能混用成"can be able to"或"can able to"
等形式.
另:can无过去式分词,但"be able to"有,为"been able to"这也就限定can不能用在完成时句中,但"be able to"可以.be able to 表示“有能力做某事”,表示某人本身具备的能力,一般可用于现在时和过去时
can 表示能够做某事,表示条件具体,可以这么做,一般只用于现在时
can（could）和be able to 的区别是：can和could只用于一般现在时和一般过去时.其他时态须用be able to.Could 不能代替 was（were）able to +动词原形.was（were）able to + 动词原形表示经过努力克服困难做成某事 1.can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.
2.be 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.
3.could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!5.在用used to谈论过去用能力做某事,但现在失去了这个能力时,我们用be able to.如：I used to be able to play the violin.6.在谈论将来有能力做某事.我们用will be able to.如：After only a few lessons,you will be able to understand basic Spanish.7.在其它动词may,might,would,want或hope后,用be able to.如：He might be able to fix your car.You should be able to taste the difference.I want her to be able to use a computer.