1. He spoke so fast that I could not follow.

2. It was so easy a question that every one of us could answer it.

3. Much to his surprise, none of them could tell the difference between them.

先说答案：这三个句子 could 都可以换为 was/were able to，因为就“能力” 而言，后者的意思包含着前者的意思。

could 只表示有这种“能力”，不表示“能力的结果”。was/were able to 既可以表示“能力”，还可以表示“结果”，即：它包含着 could 的意义。

知道了这个区别，那么，在一些不同的句子中，需要根据语义选择不同的表达。例如：

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

A. could （误） B. was able to （正）

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：“设法做了某事(managed to do sth.)”，“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)。

但是，如果有能力且做到了，那么，只能用 was able to，不能用 could，例如：

At last he was able to solve the problem.（正）

At last he could solve the problem.（误）

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

Only when the war was over could he 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.