1）can could 表示能力；可能 (過去時(shí)用could),
只用于現(xiàn)在式和過去式(could).be able to可以用于各種時(shí)態(tài).
They will be able to tell you the news soon.他很快就能告訴你消息了.
2）只用be able to
a.位于助動(dòng)詞后.
b.情態(tài)動(dòng)詞后.
c.表示過去某時(shí)刻動(dòng)作時(shí).
d.用于句首表示條件.
e.表示成功地做了某事時(shí),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.
注意：could不表示時(shí)態(tài)
1）提出委婉的請求,（注意在回答中不可用could）.
--- Could I have the television on?
--- Yes,you can./ No,you can't.
2）在否定,疑問句中表示推測或懷疑.
He couldn't be a bad man.
他不大可能是壞人.
再如：
can 與 be able to 都可以表示能力,但兩者在用法上有點(diǎn)差異：can (could)表示主觀能力,不表示意愿,它的將來時(shí)用will be able to; be able to表示主觀意愿,強(qiáng)調(diào)要克服困難去做某事.例如：
1）My grandma is over seventy,but she can still read without glasses.
2）He is able to give up his bad habits.