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could和was able to相等嗎？
■山東 劉永科
最近，有網友給我留言說：
“我學了情態動詞后，覺得對它的用法好像都明白了。可是我最近碰到一個問題，是關于could 與 was able to的用法區別的，感到情況并非如此。下面就是我遇到的問題，請問：這道選擇題，正確答案是was able to，我選could為什么不可以?”
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兩個選項，意思都講不通。因為在這個語境下，might表示過去的一種可能性，而should則表示一種義務“應該”。實際上就是考查could和was able to 區別的。做語法題不能脫離語境和句子意思，所以，我們首先要看句子想表達的是什么。兩個分句分別有at first和later兩個時間副詞，說明前后對比，表達一種結果。意為：他開始不同意我的意見，但后來我終于能夠說服他簽了協議。
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：“(過去)設法做了某事(相當于managed to do sth.)”或“成功地做了某事(succeeded in doing sth.)”。如果只表達“有能力做某事”這層意思，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，但不一定去做)。
然而，如果句子只表達第二層意思，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.（誤）
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.（誤）
但是，當句子是否定意思時，could 和 was able to 沒有差別：
I couldn’t persuade him to sign the agreement. = I wasn’t able to persuade him to sign the agreement.
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