当假设是不大可能实现时,就是虚拟条件句，常用if来引导，根据对假设的可能发生的事情是发生在过去、现在、将来总结如下：
(1) 与过去事实相反：句型：条件从句 主句
过去完成时 should(would) have 过去分词
If she had worked harder,she would have succeeded. 如果她再努力一点，她就会成功的。
The rice would not have been burnt if you had been more careful. 如果你再小心一点，米饭就不会烧焦了。
If my lawyer had been here last Saturday,he would have prevented me from going. 如果我的律师上周六来过这里，他会阻止我去的。
If he had come yesterday,I should / would have told him about it. 如果他昨天来的话，我应该/本应该把这件事告诉他。
含义：He did not come yesterday,so I did not tell him about it.
If he had not been ill and missed many classes,he would have made greater progress. 如果他没有生病，错过许多课程，他会取得更大的进步。
含义：He was ill and missed many lessons,so he did not make greater progress.
例：If the man had pointed with his first finger while he was in Brunei, everyone would have thought that he was very rude.
如果这个人在文莱的时候用他的第一个手指指着他，每个人都会认为他很粗鲁。
(2) 与现在事实相反：同现在事实相反的假设.
句型 ：条件从句 主句
一般过去时 should( would) 动词原形
If they were here,they would help you. 如果他们在这里，他们会帮助你。
If I were in your position, I would call the police immediately.
如果我处在你的位置，我会立即打电话给**。
(3) 与将来事实相反：句型：条件从句 主句
一般过去时 should 动词原形
were 不定式 would 动词原形
should 动词原形
If you succeeded,everything would be all right. 如果你成功了，一切都会好起来的。
If you should succeed,everything would be all right. 如果你成功了，一切都会好起来的。
If you were to succeed,everything would be all right. 如果你成功了，一切都会好起来的。
If I should have time tomorrow, I would certainly help you.
如果我明天有空的话，我当然会帮助你的。
(4) 虚拟条件句中的倒装：(如果省去if，则要用倒装语序)
例：Should it rain, the crops would grow better.
如果天下雨的话，庄稼会长得更好。
Were I your father, I would not give you the money.
如果我是你父亲的话，我就不会给你那钱了。
Had it not been for the captain, the ship would have sunk如果不是那船长的话，那船就会沉没了
知识拓展：
1.if (表示条件)如果 ,主句用将来时,if从句用现在时表示将来
He will come if you invite him.
如果你请他,他会来的.
2. “后退一步法”
后退一步法是指在准确地判断了该句与哪一事实相反后,按虚拟语气的后退一步法处理从句谓语动词的时态.即：在非真实条件状语从句中,谓语动词按正常情况“后退一步”.也就是：
①与过去事实相反,在从句中用过去完成时形式表示.
②与现在事实相反,在从句中用过去一般时形式表示.
③与将来事实可能相反,在从句中用过去将来时形式表示.
主句中则用情态动词would,should,could 等加一个与从句一致的动词形式.例：
⑴、If I had come her yesterday,I would have seen him.
如果我昨天来的话，我会见到他的。
⑵、If I were a teacher,I would be strict with my students.
如果我是一名教师，我会严格要求我的学生。
⑶、If it should snow tomorrow,they couldn't go out.
如果明天下雪，他们就不能出去了。
