be able to与can的区别
can/could和beableto在这两个句子里的区别，为什么不能替换？
can/could和beableto在这两个句子里的区别，为什么不能替换？
的过去式是could，但could 除了可作can的过去式外，还可表比can更委婉的语气，因此在回答由could 表示请求的问句中，不能用could，但可用can。如： —Could you lend me a hand —Of course I can.
2. 在表示现在或过去的智力、体力、技能等方面的“能力”时，can, could 常可与be able to互换。如：He could (or was able to) swim five miles when he was a boy.
3. be able to有比can更多的形式，如will be able to，have (has) been able to had been able to 等等。因此be able to常被用来表示can所不能表示的将来或完成的概念。如： They will be able to run this machine on their own in three months. 他们过三个月就能自己开这台机器了。
4. 在表示成功地做了某事时，不用could，而用was / were able to (或managed to, succeeded in doing sth )来表示。如： He was able to (or managed to) finish the job yesterday。 但是couldnt和wasnt / werent able to 常可互换。如： I couldnt / wasnt able to finish the job yesterday.
5. 与感觉动词连用时，can，could亦可与be able to互换。如： We could/were able to smell something burning in the kitchen.
can和be able to用法上的区别？
基本释义：当它们表示能力的时候，是一样的。如：
He can speak two foreign languages.
He is able to speak two foreign languages.
区别：
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:
Ill 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.
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 cant be our teacher who is knocking at the door.
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 /
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：
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
7. 一般说来, be able to 后边的动词不定式没有被动语态。如:
Im sure he is able to correct his mistakes in grammar.