1）can could 表示能力；可能 (過去時用could),
只用于現(xiàn)在式和過去式(could).be able to可以用于各種時態(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. 位于助動詞后.
b. 情態(tài)動詞后.
c. 表示過去某時刻動作時.
d. 用于句首表示條件.
e. 表示成功地做了某事時,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.