could 和 might 都可用于表示可能性，而且 might 表示的可能性较小。couldn't 表示事情不可能发生，有较强的事实迹象，而 might not 用来表示事情有可能不发生，基本上就是凭空想象，所以要选用哪个，要根据上下文来判断；could 还是 can 的过去式，所以 couldn't 还可以表示过去没能力做某事。
一、用 couldn't 或 might not 完成下面的句子
1. When you were a child, you could swim from here to the little island in the middle of the river, _____ you? At least, that's what your father told me.
2. She _____ possibly be the winner of the talent show! The other acts were much better than hers.
3. Frank and Sarah _____ get tickets to the concert. The concert was sold out a little over an hour after tickets went on sale.
4. I heard that band is really popular, and tickets sell out quickly. You _____ get tickets if you wait too long.
5. That concert has been sold out for weeks. You _____ get tickets even if you knew the band personally. It's impossible!
6. It _____ be a bad idea to get a car alarm for your new sports car. New cars tend to attract thieves.
7. That _____ possibly be Mr. Jones. He's lost so much weight that he looks like a completely different person.
8. You _____ do the job if you didn't speak Arabic fluently.
9. Jane and Bill _____ have gotten the invitation to the party. Perhaps, that's why they didn't show up. I doubt they would actually skip your birthday party - they're your best friends!
10. I _____ have left my keys at Simon's house. I wouldn't have been able to drive home if I had done that. They must be somewhere here in the apartment. Let's keep looking.
二、答案、参考翻译及解析
1. couldn't
当你还是个孩子的时候，你可以从这里游到河中央的小岛上，是吗？至少，你父亲是这么跟我说的。
解析：could 的否定式是 couldn't 或 could not，所以选 couldn't.
2. couldn't
她不可能是才艺表演的获胜者！其他的表演比她的好得多。
解析：从 The other acts were much better than hers 表明，前面的事实是不可能的，所以选 couldn't.
3. couldn't
弗兰克和莎拉买不到音乐会的票。音乐会在售票后一个多小时就卖完了。
解析：这里的 could 是 can 的过去式，表示过去的一种能力，所以选 couldn't.
4. might not
我听说那个乐队很受欢迎，票卖得很快。如果你等得太久，你可能买不到票。
解析：从条件句 if you wait too long 可以看出，能否买到票只是一种可能性。
5. couldn't
那场音乐会已经卖完好几个星期了。即使你认识乐队，你也买不到票。这不可能！
解析：从 has been sold out 可以看出票已经卖完了，买到票是不可能的事；这个与题4的 sell out 的区别是，题4的 sell out 只是强调一般的事实，而题5强调的是 has been sold out，也即售完的事实。
6. might not
给你的新跑车装个汽车报警器也许不是个坏主意。新车容易吸引小偷。
解析：主意本来就是主观的东西，所以选 might not.
7. couldn't
那不可能是琼斯先生。他瘦了很多，看起来完全不一样了。
解析：从后半句的语气来判断应该用 couldn't.
8. couldn't
如果你阿拉伯语讲得不流利，你就不能胜任这项工作。
解析：这里的 could 是 can 的过去式，表示过去的一种能力，might 没有表示能力的用法，所以选 couldn't.
9. might not
简和比尔可能没有收到参加聚会的邀请。也许，这就是他们没有出现的原因。我怀疑他们真的会忽略你的生日聚会-他们是你最好的朋友！
解析：从 Perhaps 判断前面是一种猜测，所以选 might not.
10. couldn't
我不可能把钥匙留在西蒙家。如果我那样做的话，我就不能开车回家了。他们一定在公寓的某个地方。我们继续找吧。
解析：从后面的句意判断，前面的事情是不可能发生的，所以选 couldn't.
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