be able to和could

题目

be able to和could
表示过去实际上可以做到的事时只用was/were able to不用could
那这个怎么解释I was able to speak English中的 was able to 可以换成could
求精简的解释,注意针对问题回答

答案

一般泛泛地讲“某人过去能做什么”,用could
你的那个句子还是用could 较好
I could speak English
具体某一次做成什么,用was able to
如：
Two hours later,he was able to swim across the river.
否定句中可以互换
He was so drunk that he couldn't/ wasn't able to find the door.