虚拟语气的三种时态基于三种不同的情况：
与现在事实相反，从句的谓语动词用一般过去时did/were，主句的谓语动词用would/should/could/might+do
与过去事实相反，从句的谓语动词用过去完成时had done,主句的谓语动词用would/should/could/might+have done
与将来事实相反，从句的谓语动词用did/were或者were to do或者should do,主句的谓语动词用would/should/could/might+do
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例句
①If I were you ,I would consider their advice into account.
如果我是你的话，我会重新考虑他们的意见。（与现在事实相反）
②You would not have caught a cold if you had put on more clothes.
如果你多穿了些衣服，你就不会得感冒了。（与过去事实相反）
③If I were to / should do it ,I would do it well.
如果我做这件事，我将用不同的方法做。（与将来事实相反）
规律总结：从句都往过去推一个时态
虚拟条件句的倒装形式:主要指if的省略。在省略时,句中的had或were调到句首。(只有were,had,should能倒装到句首。
如:If we had made adequate preparation,we might have succeeded.
Had we made adequate preparation, we might have succeeded.
含蓄虚拟条件句:指不通过i从句明显表现出来,而是隐含在副词,介词短语或上下文中。常有:otherwise,or, with(如果有了),without(要是没有),but for(要不是)
under(要是在…的情况下),would rather(宁愿)等
如: He must have had an accident, or he would have been here then.
But for your sister's help,you wouldn't have finished your college course.
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错综时间条件句
如:If he had acted on my advice,he would be teacher now.
If he had received 6more votes,he would be our chairman now.
虚拟语气在其他从句中的使用,从句中的动词用法有以下几种:
1.if only,wish,as if,as though, but for if后的句子中,
从句谓语动词用过去时,表示与现在的实际情况相反的希望。
过去完成时表示与过去实际情况 相反的愿望