He could speak English.与 He was able to speak English.两句英文句子的区别在哪里?

He could speak English means he can speak English, but not necessarily speak it well.
He was able to speak English means he was able to speak it in the past, but perhaps he cannot speak well now.
理解英文绝对不是用中文就能理解明白的.不信就算了.