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.