could和was able to相等吗？

could和was able to相等吗？

最近，有网友给我留言说：

“我学了情态动词后，觉得对它的用法好像都明白了。可是我最近碰到一个问题，是关于could 与 was able to的用法区别的，感到情况并非如此。下面就是我遇到的问题，请问：这道选择题，正确答案是was able to，我选could为什么不可以?”

He didn’t agree with me at first, but I ________ persuade him to sign the agreement later. 　　

A. could        B. might         C. should       D. was able to　　

本句中might和should两个选项，意思都讲不通。因为在这个语境下，might表示过去的一种可能性，而should则表示一种义务“应该”。实际上就是考查could和was able to 区别的。做语法题不能脱离语境和句子意思，所以，我们首先要看句子想表达的是什么。两个分句分别有at first和later两个时间副词，说明前后对比，表达一种结果。意为：他开始不同意我的意见，但后来我终于能够说服他签了协议。

could表示“(过去)有能力做某事”，was able to 除了这层意思外，还有一个重要的意思，就是：“(过去)设法做了某事(相当于managed to do sth.)”或“成功地做了某事(succeeded in doing sth.)”。如果只表达“有能力做某事”这层意思，那么这两个可以互换，而意思保持不变，例如：　　

He was very strong and he could (=was able to) lift that big stone. 他很强壮，他能够举起那块大石头(意思是他有这个能力，但不一定去做)。　　

然而，如果句子只表达第二层意思，那么就只能用 was able to，不能用could。例如：　　

He was puzzled at a math problem, but at last he was able to solve it with the help of his teacher.（正）　　

He was puzzled at a math problem, but at last he could solve it with the help of his teacher.（误）　　

Only when the war was over was the professor able to get happily back to work.（正）　　

Only when the war was over could the professor get happily back to work.（误）　　

但是，当句子是否定意思时，could 和 was able to 没有差别：　　

I couldn’t persuade him to sign the agreement. = I wasn’t able to persuade him to sign the agreement.