作业帮be able to 和can 有什么区别,有例题讲解He was so good a swimmer that he___ swim to the river bank when the boat sank.用哪个,为什么
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be able to 和can 有什么区别,有例题讲解
He was so good a swimmer that he___ swim to the river bank when the boat sank.
用哪个,为什么
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.
can用于表示“能力”时是指现在的能力,过去或将来的能力通常用was/were able to或will/shall be able to表示.
eg. She was able to go to school yesterday.
Soon she will be able to swim a quarter of a mile.
Can的过去式形式could也可用于表示泛指过去的能力.如：
I could read when I was four.
但不能用于表示特定的某一过去能力,如不能说：
*He could swim halfway before he got tired.
He was able to swim halfway before he got tired.
上述这种用法的区别不存在于否定句中.在否定句中,couldn’t与was/were not able to 可以互换使用.
eg. I couldn’t see him again before he left.
I wasn’t able to see him again before he left.
另外,要说一个阶段延续至今的能力,可用have/has been able to表示.
eg. John has been able to swim for many years.
注意的一点是：can也可以用于表示“许可”.它可以表示现在许可或将来许可,过去许可用could表示,此时一律不能用be able to的某种形式替换.
个人认为选can,can表示有能力做的到 如：can you play the piano?
be able to 用于构成将来时，完成时，动词不定式 如：she`d love to be able to play the piano.
can和be able to都表示有可能有机会。如can you come on sunday?=Are you able to come on sunday?
应用 be able to。 因为由was可看出此句是过去式，而can只能用在现在时与将来时之中，唯有be able to用在完成时与过去时之中。区别就在此吧。