how terrible it was that an earthquake struck our city at midnight yeaterday.but luckily we ( )escape from our apartment
a were able to
b could
我选a
我想问的是
was/were able to 与could的区别
还有be able to和can的区别
还有这两组之间的区别

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。


参考文献：baidu