基本釋義：當它們表示能力的時候，是一樣的。如：
He can speak two foreign languages.
He is able to speak two foreign languages.
區別：
1. can只有兩種時態, 即can 和could, 而be able to 有多種時態, 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.
2．表示過去通過努力終於做成了某事要用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,而且只有兩種形式,即:can, could．be able to 則主要指具體做到了某件事的能力，其形式主要是靠be 發生變化，所以形式比can 多．
可以說：I can swim. I am able to swim.
但是不能說：All the people could escape from the big fire in time.
只能說：All the people were able to escape from the fire in time.
3．can能表猜測，be able to不能。can的這種用法主要用於疑問句和否定句。
Somebody is knocking at the door. Who can it be?
It can’t be our teacher who is knocking at the door.
4．can 可以表 「允許」, 與may可以互換, 此時不能用be able to 代替。如：
You can attend the party.
5. can 是情態動詞，只能用作謂語成分，be able to是普通動詞短語，既可以作謂語，也可以作非謂語。如：
How long to be able to communicate with the foreigners in fluent English!
6. be able to 後邊接動詞不定式表示一種實際情況時，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. 一般說來, be able to 後邊的動詞不定式沒有被動語態。如:
I’m sure he is able to correct his mistakes in grammar.