Learner's definition of COULD
[modal verb]
1
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used as the past tense of can能够，可以，会（用作can的过去式） -
When I was younger I could run fast, but I can't run fast now.我年轻时能跑得很快，但现在不行了。
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Years ago you could buy a record album for a quarter.多年前，花上二十五美分就能买到一张唱片。
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The car cost more than I could afford, so I bought a cheaper model.这款车的价格超出了我的支付能力，所以我买了较便宜的一款。
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From where we stood, we could see for miles.从我们所站的位置，我们能够远眺数英里。
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She could be very rude at times. [=she was sometimes very rude]她有时会很粗鲁。
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He never could quite fit in with the group.他从来没能融入这个团队。
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She said we could do whatever we wanted. [=she said, “You can do whatever you want.”]她说我们想干什么都可以。
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How could something like this happen?怎么会发生这种事？
2
a —
used to say that something is possible可能，也许（用于表示某事可能会发生） -
You could [=might, may] be making the biggest mistake of your life!你可能会犯下你一生最大的错误！
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This could be our only chance to get out of here.这可能是我们离开这里的唯一机会。
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His recovery could take months.他的康复可能需要数月的时间。
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This medicine could help your cold.这种药也许能治疗你的感冒。
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The evidence is not conclusive, but he could (very well) be right.虽然证据并不确凿，但他很可能是对的。
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This news could mean trouble.这条新闻可能会引起麻烦。
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Did you think he could still be alive?你认为他有没有可能还活着？
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That kind of thing could happen to anyone.那种事情有可能发生在任何人身上。
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They could still succeed, although it's not likely.虽然他们成功的可能性不大，但还是有可能的。
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I think he could be trying to cheat us.我认为他可能会试图欺骗我们。
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Do you think he could [=can] really be serious?你认为他会动真格的吗？
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Could this be our ride? [=is this our ride?]这是我们要搭的车吗？
b —
used with have to say that something was possible but did not actually happen本可能，差点就（与have连用，表示某事本可能发生但实际上未发生） -
You could have been seriously hurt.你本有可能受重伤。
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The accident could have been worse.这次事故原本可能更严重。
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The movie could have been better. [=the movie wasn't very good]这部电影本该更好。
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They could have succeeded if they had worked harder.如果之前更加努力，他们可能已经成功了。
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The error could have lost the game for them.这个差错险些使他们输掉这场比赛。
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We could just as easily have fixed the engine ourselves.我们本来可以自己轻易修好这台发动机。
c —
used to talk about something that is not possible but that is hoped or wished for要是能…就好了（用于表示希望某事发生，尽管实际上不可能） -
We would go if only we could—but unfortunately we can't.要是能去，我们就会去，但遗憾的是我们去不了。
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We would have gone if only we could have—but unfortunately we couldn't.要是能去，我们就去了，但遗憾的是我们去不了。
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I wish I could fly! = (formal) Would that I could fly!真希望我能飞起来！
3
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used in speech to make a polite request or suggestion能否，可不可以（用于口语，表示礼貌地提出请求或建议） -
Could you please pass me the salt?请把盐递给我好吗？
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Could [=Can] I leave a little early today?我今天早点儿走行吗？
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If you could come early, we would be pleased.如果您能早点儿来，我们会很高兴。
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“Could I do it tomorrow instead of today?” “Yes, you can.”“我今天不做明天再做，行吗？”“可以。”
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Could I get you a cup of coffee? [=do you want a cup of coffee?]您要喝杯咖啡吗？
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If tape doesn't work, you could try glue.如果胶带不行，您可以试试用胶水。
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“Where can I stay?” “Well, you could always stay with me.”“我住哪儿？”“嗯，你随时可以和我住。”
4
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used to say that you are annoyed by something that was or was not done本该，应该（用于对做过或未做某事表示恼怒） -
He could have at least paid for dessert!他至少该把甜点钱付了嘛！
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Why are you always late? You could try being on time for once!你为什么总是迟到？你试着准时一回总可以嘛！
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If she wasn't going to make it, she could have called.如果她不打算出席，也该打个电话呀。
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You could at least tell me why you were late!你至少应该告诉我你为什么迟到！
5
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used in statements that express a strong emotional reaction真想（用于表达强烈的情感） -
I could have died of embarrassment! [=I was very embarrassed]我窘得要死！
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I could just kill him! [=I am very angry at him]我真想杀了他！
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When she told me I'd won, I could almost have hugged her! [=I felt like hugging her]当她告诉我我赢了的时候，我真想拥抱她！
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I'm so frustrated I could scream! [=I feel like screaming]我沮丧透了，真想大喊大叫！
6
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used in statements that describe something as very bad, good, etc.不能更…了（用于表示某事很好、很差等） -
What could be better than this?还有什么会比这个更好呢？
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Registering could not be simpler. [=registering is very simple]注册简直太简单了。
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I could hardly be more pleased [=I'm very pleased] with the way things are going.我对事态的发展非常满意。
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The situation couldn't be worse/better. [=the situation is as bad/good as it can be]情况糟透/好极了。
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I couldn't be happier [=I'm very happy] with the way our new kitchen looks.我简直太喜欢我们新厨房的样子了。
could care less
— see 2care
could do with
◊ If you could do with something, you need it or would be helped by getting or having it.需要；想要