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“情態動詞用法”問答
■本站特約作者 劉永科
【問題一】在人教版必修一第2單元有這樣一個句子：By the 1600’s, Shakespeare was able to make use of a wider vocabulary in his writing than ever before.這句話能否用could代替was able to？
在必修三第2單元還有一個句子：Perhaps with a discount and a new sign he could win his customers back. 這句話能否用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. 這句話用could為何不可以？
這些問題，實際上就是could和was able to的區別。could表示“（過去）有能力做某事”，was able to 除了這層意思外，還有一個重要的意思，就是：“設法做了某事（managed to do sth.）”，“成功地做了某事（succeeded in doing sth.）”，往往暗示經過努力才具備的能力。如果只表達“有能力做某事”這層意思，而不表達是否實施，那么這兩個可以互換，而意思保持不變，例如：
He was very strong and he could（=was able to）lift that big stone. 他很強壯，他能夠舉起那塊大石頭。（意思是他有這個能力，是否實施不得而知）
但是，如果是第二層意思，即“能夠設法做了某事，成功地做了某事”，那么只能用was able to，不用could。請比較：
At last he could solve the problem.（誤）
At last, he was able to solve the problem.（正）
Only when the war was over could he get happily back to work.（誤）
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. （誤）
It was a very big fire, but most people were able to escape from it.（正）
但是，在否定句中兩個可以互換，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.
在與感知動詞連用時，如recognize, see, understand, realize等，用于肯定句或否定句都可，兩者沒有區別。
The little boy could recognize his mother.= The little boy was able to recognize his mother.
現在回到前面的問題。
1. By the 1600’s, Shakespeare was able to make use of a wider vocabulary in his writing than ever before.這句話能否用could代替was able to？
這句話could不能代替was able to，是因為在其作品中使用更大范圍的詞匯量，莎士比亞已經成功地做到了。
2. Perhaps with a discount and a new sign he could win his customers back. 這句話能否用was able to代替could？
我們找出課文看看，王鵬對他餐館的未來充滿了希望，他采取打折和換用新的牌子的方式招徠顧客，這可能喚回顧客，但僅是一種可能，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. 這句話用could為何不可以？
本句前后兩個分句是轉折關系。他一開始不同意，但是我后來還是能夠說服他簽了合同，表示結果，故用was able to。
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