作业帮can could be able to
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can could be able to
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.
再如：
can 与 be able to 都可以表示能力,但两者在用法上有点差异：can (could)表示主观能力,不表示意愿,它的将来时用will be able to; be able to表示主观意愿,强调要克服困难去做某事.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.
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.
再如：
can 与 be able to 都可以表示能力,但两者在用法上有点差异：can (could)表示主观能力,不表示意愿,它的将来时用will be able to; be able to表示主观意愿,强调要克服困难去做某事.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.
can could be able to
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