虚拟条件句的核心在于设想一种不真实的情况，其可能性极低或根本无法实现。常用"if"引导，按时间和现实状况分为不同类型：
(1) 过去事实的虚拟条件：
用于假设过去的事与实际发生的事相反。
句型：条件从句 主句
使用过去完成时 should/would have + 过去分词
例如：
If she had worked harder, she would have succeeded. 如果她当时更努力一些，她就会成功。
The rice would not have been burnt if you had been more careful. 如果你当时更小心一些，米饭就不会烧焦。
If my lawyer had been here last Saturday, he would have prevented me from going. 如果我的律师上周六在这里，他会阻止我离开的。
这些句子表明，实际情况是：她没有更加努力，米饭被烧焦了，我的律师未能到场。
例子：If the man had pointed with his first finger while he was in Brunei, everyone would have thought that he was very rude. 如果那个在文莱的人当时用食指指人，大家会认为他很粗鲁。
(2) 现在事实的虚拟条件：
用于假设现在的事与实际情况相反。
句型：条件从句 主句
使用一般过去时 should/would + 动词原形
例如：
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. 如果我在你的位置，我会立即打电话报警。
(3) 将来事实的虚拟条件：
用于假设将来的事与实际情况相反。
句型：条件从句 主句
使用一般过去时 should + 动词原形
或者 were + 不定式 would + 动词原形
例如：
If you succeeded, everything would be all right. 如果你成功了，一切都会好起来。
If you should succeed, everything would be all right. 如果你成功了，一切都会好起来。
If you were to succeed, everything would be all right. 如果你成功了，一切都会好起来。
If I should have time tomorrow, I would certainly help you. 如果我明天有时间，我肯定会帮你。
(4) 虚拟条件句中的倒装：
省略"if"时，需要使用倒装语序。
例如：
Should it rain, the crops would grow better. 如果下雨的话，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，我就不会给你那笔钱了。
Had it not been for the captain, the ship would have sunk. 如果不是船长，船就会沉没了。
知识拓展：
1. 表示条件的“if”句型：当条件是未来的可能性时，主句用将来时，而条件从句用现在时。
例如：
He will come if you invite him. 如果你邀请他，他会来的。
2. “后退一步法”：用于准确判断虚拟语气的时态。即根据假设情况的时间点，调整从句中的动词时态。
例如：
① 与过去事实相反：从句使用过去完成时。
If I had come here yesterday, I would have seen him. 如果我昨天来的话，我会见到他的。
② 与现在事实相反：从句使用过去时。
If I were a teacher, I would be strict with my students. 如果我是一名老师，我会严格要求我的学生。
③ 与将来事实相反：从句使用过去将来时。
If it should snow tomorrow, they couldn't go out. 如果明天下雪，他们就不能出去了。
