英语中常接助动词 have 的情态动词有 could，should和 would 等，它们都是假设性的，用来谈论过去没有发生过的事情，通常缩写为 could've，should've 和 would've。 一、could have 1. 表示某人过去可以做某事，但实际上没有做，通常意为“本来可以，本可以”，相当于 be (was/were) able to do sth，例如：I could have stayed up late, but I decided to go to bed early.我本来可以熬夜的，但我决定早点睡觉。解析：此句相当于 I was able to stay up late, but I didn't.I could have phoned, but I preferred to talk to you personally.我本来可以打电话的，但我更愿意亲自和你谈谈。2. 表示某事过去可能发生，但没有发生，通常意为“本来可能”，例如： You could have dropped all the plates. Be more careful next time.你本来可能会弄掉所有的盘子。下次小心点。解析：此句相当于 There was a possibility of you to drop all the plates, but it didn't happen.He could have broken his leg. It's good you were there to catch him.他本来可能摔断了腿。很高兴你在那里抓住了他。I could have been there on time if I had left home earlier.如果我早点离开家，我本可以准时到那儿的。3. 表示不确定自己说的是不是对的，通常意为“可能”，例如： I'm not sure why he's so late. He could have missed the bus.我不知道他为什么这么晚。他可能没赶上公共汽车。They didn't know why the shipment hasn't arrived yet. They said it could have gone to the wrong address.他们不知道为什么这批货还没到。他们说可能去错地址了。I don't know why she didn't read it. That page could have been missed.我不知道她为什么不看。那一页可能被遗漏了。从第二三点可以看出，当 could have 表示推测时，跟 might have 的用法是一样的，虽然它们所表达的可能性高，但相比于 could，might 的可能性更小，例如： I might have moved out, but I didn't want to leave my family.我可能搬走了，但我不想离开我的家人。二、should have1. should have + 过去分词，表示某事是个好主意或是本来应该去做的，但没去做。I should have studied harder!我应该更努力学习的！解析：此句相当于 I didn't study very hard and so I failed the exam. I'm sorry about this now.I should have gone to bed early.我应该早点睡觉的。2. shouldn't have + 过去分词，表示某事不是一个好主意，但还是做了，例如：I shouldn't have eaten so much cake!我不该吃这么多蛋糕！I shouldn’t have said that.我不该那么说。3. should have + 过去分词，表推测，表示如果一切正常的话，那么某事已经发生，常与 by now 连用，例如：He should have received it by now.他现在应该已经收到了。John should have finished work by now.约翰现在应该已经完成工作了。
三、would haveWould have + 过去分词，表示想做却没做的事，意为“本来想要，本来要”，例如：I would have gone to the party, but I was really busy.我本来要去参加聚会的，但我真的很忙。I would have called you, but I didn't know your number.我本想打电话给你，但我不知道你的电话号码。
