“情态动词用法”问答

■本站特约作者  刘永科

【问题一】在人教版必修一第2单元有这样一个句子：By the 1600’s, Shakespeare was able to make use of a wider vocabulary in his writing than ever before.这句话能否用could代替was able to？

在必修三第2单元还有一个句子：Perhaps with a discount and a new sign he could win his customers back. 这句话能否用was able to代替could？

He didn’t agree with me at first, but I was able to persuade him to sign the agreement later. 这句话用could为何不可以？

这些问题，实际上就是could和was able to的区别。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.）”，往往暗示经过努力才具备的能力。如果只表达“有能力做某事”这层意思，而不表达是否实施，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，是否实施不得而知）

但是，如果是第二层意思，即“能够设法做了某事，成功地做了某事”，那么只能用was able to，不用could。请比较：

At last he could solve the problem.（误）

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

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

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

It was a very big fire, but most people could escape from it. （误）　　

It was a very big fire, but most people were able to escape from it.（正）　　

但是，在否定句中两个可以互换，而意思相同：

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

They couldn't finish the task in such a short time.= They were not able to finish the task in such a short time.　　

在与感知动词连用时，如recognize, see, understand, realize等，用于肯定句或否定句都可，两者没有区别。　　

The little boy could recognize his mother.= The little boy was able to recognize his mother.

现在回到前面的问题。

1. By the 1600’s, Shakespeare was able to make use of a wider vocabulary in his writing than ever before.这句话能否用could代替was able to？

这句话could不能代替was able to，是因为在其作品中使用更大范围的词汇量，莎士比亚已经成功地做到了。

2. Perhaps with a discount and a new sign he could win his customers back. 这句话能否用was able to代替could？

我们找出课文看看，王鹏对他餐馆的未来充满了希望，他采取打折和换用新的牌子的方式招徕顾客，这可能唤回顾客，但仅是一种可能，能否成功不得而知。故只能使用could。

3. He didn’t agree with me at first, but I was able to persuade him to sign the agreement later. 这句话用could为何不可以？

本句前后两个分句是转折关系。他一开始不同意，但是我后来还是能够说服他签了合同，表示结果，故用was able to。

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