He spoke so fast that I could not follow.

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

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

這類個例句中的could是表示過去的能力嗎?她們能否用 was able to來替代?

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.

先說答案：這三個句子 could 都可以換為 was/were able to，由于就“能力” 而言，后者的意思包括著前者的意思。

could 只表示有這種“能力”，不表示“能力的結果”。was/were able to 既能夠表示“能力”，還可以表示“結果”，即：它包括著 could 的意義。

了解了這個不同，那樣，在一些不一樣的句子中，需要依據語義選擇不一樣的表達。比如：

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

A. could （誤）B. was able to （正）

could 表示“有能力做某事”，was able to 除去這層意思外，還有一個要緊的意思，就是：“設法做了某事”，“成功地做了某事 ”。假如只表達“有能力做某事”這層意思，那樣這兩個可以互換，而意思維持不變，比如：

He was very strong and he could lift that big stone. 他非常強壯，他可以舉起那塊大石頭。

但，假如有能力且做到了，那樣，只可以用 was able to，不可以用 could，比如：

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

At last he could solve the problem.（誤）

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.（誤）

但，當句子是不是定意思時，could 和was able to 沒差別：

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