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.
注意：could不表示时态
1）提出委婉的请求,（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,疑问句中表示推测或怀疑.
He couldn't be a bad man.
他不大可能是坏人.