要掌握be able to的具体用法就必须搞清can和be able to二者区别,这两个词词义是一样的,有时可以互用,但它们之间用法也是有区别的,什么情况下用can什么情况下用be able to这里有讲,http://www.***.cn/g1/lesson/p/p1211/?c=vip744928.在这里我把二者的区别和be able to具体用法详细发来,供您参考.
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.
