情态动词 + 动词完成式即“ 情态动词 + 动词完成式即“情态动词 + have + done”，表示对过去行为或动作进行推测、评论或判断。
1、表示与现在事实相反的情况：
主语+should/would/could/might+do+其他+
if+主语+did/be(were)+其他
例：
1.If I were you, I would take an umbrella.如果我是你，我会带把伞。（事实：我不可能是你）
2.If I knew his telephone number, I would tell you. 如果我知道他的电话号码，我就会告诉你。（事实：不知道）
3.If there were no air or water, there would be no living things on the earth. 如果没有水和空气，地球上就不会有生物。(事实：地球上既有空气也有水)
4.If I had any money with me, I could lend you some. 如果我带钱了，我就会借给你些。（事实：没带钱）
5.If he studied harder, he might pass the exam.如果他再努力些，就能通过考试了。（事实：学习不用功）
肯定：主语+should(ought to)/would/could/might/need + have done+其他+
if+主语+had done +其他
否定：主语+shouldn't(oughtn't to)/wouldn't/needn't + have done+其他+
例：
1. If I had got there earlier, I should/could have met her. 如果我早到那儿，我就会见到她。（事实：去晚了）
2.If he had taken my advice, he would not have made such a mistake. 如果他听我的劝告的话，就不会犯这样的错误了。（事实：没有听我的话）
（可能相反或可能性很小）
主语+should/would/could/might+do+其他
if+主语+did/should/were to do+其他
例：
1.If he should come here tomorrow, I should/would talk to him. 如果他哪天来这儿的话，我就跟他谈谈。 （事实：来的可能性很小）
2.If there were a heavy snow next Sunday, we would not go skating. 如果下周日下大雪，我们就不能去滑冰了。（事实：不知能否下雪）
3.If she were to be there next Monday, I would tell her about the matter. 如果她下周一来这儿的话，我就会告诉她这件事的始末。
在表示建议、命令、要求等含义的宾语从句，谓语动词常用虚拟语气，"should＋动词原形"构成，should 可省略。
a、表示与现在事实相反的愿望，谓语动词用过去式
eg. I wish I had your brains.我希望我有你那样的头脑。(事实：我根本比不上你)
b、表示与过去事实相反的愿望，谓语动词：had+done(动词过去分词）
eg:.I wish I had known the truth of the matter.我希望我原来知道这件事的真相。（事实：原来不知道）
c、表示将来难以实现的愿望
谓语动词：should/would + 动词原形eg. I wish I should have a chance again.我希望我还能有一次这样的机会。（事实：很难再有这样的机会了）
(注：if only和even if也有相同用法）
一想要(desire)二宁愿(prefer)三命令(order. command)四建议(advise. suggest. propose)五要求(demand. require. request. desire.insist)中，无论主句谓语动词为何种时态，从句的谓语动词都用：“should + 动词原形”或只用“动词原形”。
如 He suggested that we (should) take the teacher’s advice
He insisted that we (should) take the teacher’s advice
He demand that we (should) take the teacher’s advice
He ordered that we (should) take the teacher’s advice
注：insist如果翻译成坚持某种动作才用虚拟语气翻译成坚持某种观点就不用虚拟语气。
如：He insist he is a student. 他坚持说他是个学生。
这个语句表示的是事实，因此在这个语句中不能使用虚拟语气。
suggest意为“建议”才用虚拟语气，意为“暗示”则不用虚拟语气。
如： His face suggests that he looks worried .他的表情暗含着他很担心。
这个句子本身是事实，因此它就没有用到虚拟语气。
③表情绪.观点的形容词或名词也要用虚拟语气.如:necessary. important. impossible. natural. strange. surprising.
funny. right. wrong. better. a pity等。
句型：It is.......that 结构后的主语从句，从句的谓语动词都要用 should+原型 或只用动词原型。
一、在虚拟语气中的用法
1. should have done 表示“过去本应该做某事却未做。”其否定结构 shouldn't have done 表示 “过去本不该做某事却做了。”
2. ought to have done 表示“过去本应该做某事却未做。”其否定结构 oughtn't to have done 表示“过去本不该做某事却做了。”
3. need have done 表示“过去本有必要做某事却未做。”其否定结构 needn't have done 表示“过去本没必要做某事却做了。”
4. could (不能用 can) have done 表示“过去本能够做某事却未做。”注意：其否定形式 couldn't have done 没有虚拟语气的用法，couldn't have done 只能表推测，相当于 can't have done，意为：“过去不可能做了某事。”could have done 的否定形式应用wouldn't have done。注意：could have done 还可以表示推测。
5. might (不能用 may) have done 表示“过去本可以做某事却未做。” 注意：其否定形式 might not have done 没有虚拟语气的用法，might not have done 只能表推测，相当于 may not have done，意为：“过去可能没有做某事。”might have done 的否定形式应用wouldn't have done。注意：might have done 还表示推测。
二、表推测的用法
1. must have done 表示对过去某事的肯定猜测，译为：过去肯定做了某事。不存在 mustn't have done 的形式。其否定或疑问形式须用can(could) 来表示. 例如： Since the road is wet, it must have rained last night. 既然路是湿的，那昨天晚上肯定下雨了。 He can't have missed the way. I drew him a map. 他不可能迷路。我（当时）给他画了张图。 “The dictionary has disappeared. Who could have taken it?”词典不见了，（过去）谁可能把它拿走了？
2. may / might have done 表示“过去可能做了某事”。may 比 might表示的可能性在说话人看来稍大些。may/might not have done表示“过去可能没有做某事。”例如： I can't find my keys. I may / might have left them at the school yesterday. 我找不到我的钥匙了。我可能昨天把他们落在学校了。John may/might not have passed the exam; he looks very sad.约翰可能没有通过考试。他看起来很忧伤。注意：may/might have done表推测不能用于疑问句中。
3. can/could have done 表推测一般用在否定句和疑问句中，表示不相信或怀疑的态度。 Can/Could he have passed the exam?他可能通过了考试吗？ I think that he couldn’t/can't have gone abroad. I saw him just now.我认为他不可能出国了。我刚才还看见他了。 注：表示推测过去某动作发生的可能性时，就表示的可能性程度而言，must 最大，could 其次，may 更次之，might 最小。例如：“I wonder how Tom knew about your past.” “我想知道汤姆是怎么知道你的过去的。”“He must / could / may / might have heard of it from Mary.” “他肯定/很可能/可能/兴许（没准儿）已从玛丽那儿听说此事了。”
