与现在事实相反 If I (we, you, he, they) +动词过去式(be的过去式用were) I (we) should/you would/he would/they would+动词原形
与过去事实相反 If I (we, you, he, they) + had+过去分词 I (we) should/you would/he would/they would+ have+过去分词
与将来事实可能相反 If I (we, you, he, they) +动词过去式(be的过去式用were) I (we) should/you would/he would/they would+动词原形
[注]美国英语的结果主句，不管什么人称，皆常用would.
1)与现在事实相反的非真实条件句，条件从句的谓语用动词的过去式(be的过去式用were)，结果主句的谓语用should(第一人称)或would(第二、三人称)+动词原形。如：
(3)If we left now, we should arrive in good time. 假如我们现在就走的话，我们就会及时到达。
(4)If I were you, I would refuse the money. 假如我是你的话，我就不会要那钱。(would代替了should)
(5)Even if he had the money, he wouldn't buy it. 他即使有钱也不会买它。
2)与过去事实相反的非真实条件句，条件从句的谓语用had+过去分词，结果主句的谓语用should(第一人称)或would(第二、三人称)+ have+过去分词。如：
(6)We would've dropped by if we had had time. 假若我们有时间的话，我们就会顺道拜访了。
(7)I wouldn't have known what these were for is I hadn't been told. 假如别人不告诉我，我就不会知道这些东西是干什么的了。
但虚拟式were也可表与过去事实相反。如：
(8)If I were not busy, I would have come. 假如我不忙，我就会来了。
表过去时间的假设亦可用虚拟式should have+过去分词，这种虚拟式所表的动作并非完全不可能发生，它只是表示一种偶然性，并常和直陈语气连用。但也可与虚拟语气连用。如：
(9)You can imagine what would have happened to her if she should have told the truth. 你可以想像，如果她道出了真情，她的遭遇会如何。
美国大众语常用would have+过去分词表示与过去事实相反的假设。如：
(10)If this sleeplessness would have been allowed to go on, she would have collapsed. 这种失眠如果任其发展，她就会垮掉的。
3)与将来事实可能相反的非真实条件句，条件从句的谓语用动词的过去式(be的过去式用were)，结果主句的谓语用should(第一人称)或would(第二、三人称)+动词原形。如：
(11)If you dropped the glass, it would break. 假如你把玻璃杯掉在地上，它会打碎的。
(12)If you lived there for a while, you'd change your mind about that place. 假如你在那里住上一段时间，你就会改变对那地方的看法了。
[注]如无上下文，下面句中的rained有歧义，可能是表与将来事实可能相反的虚拟语气，也可能是表过去习惯的直陈语气：If it rained we would stay indoors and read. 如果下雨，我们就待在家里看书。
在与将来事实相反的条件句中，其条件从句的谓语亦可用were to+动词原形。这种虚拟时式比较正式，多用于书面语中，其假想性很强，实现性甚小。如：
(13)If he were to come, what should we say to him? 假如他来了，我们对他说什么呢?
在与将来事实相反的条件句中，其条件从句的谓语还可以用should+动词原形。这种形式并不强调意愿，只强调一种有偶然实现的可能性。如：
(14)If he should see me, he would know me. 假如他看见我，就会认识我。
这种形式往往有不为人所欢迎的含义。如：
(15)If he should go away, I should be grieved. 他如离去，我就会感到悲伤。
有时可用would代替should，以免与结果主句中的should重复。如：
(16)If you would be interested, I should be very glad to send you a copy of my book. 你如感兴趣，我将高兴寄给你一册我的书。
在这种条件句中，结果主句常用直陈语气或祈使语气。如：
(17)If the train should be late, what will you do? 如果火车晚点了，你将怎么办?(结果主句用疑问句)
(18)If we should fall in this, we are ruined. 我们这个计划如果失败，我们就倒霉了。(结果主句用直陈语气)
(19)If you should meet Henry, tell him I want to see him. 假如你见着亨利，告诉他我要见他。(结果主句用祈使语气)
关于过去式、should+动词原形与were to+动词原形这三种虚拟式的可能性孰大孰小的问题，语法家们认为它们的可能性虽都不大，但相比较，were to+动词原形的可能性最小，should+动词原形次之，过去式更次之。试比较：
(20)If the ship left at noon, it would pass through the canal between 2 and 2:30 p.m. 这艘船如果中午启航，下午2时至2时半之间将穿过运河。(有可能，但不大)
(21)If the ship should leave at noon, it would pass through the canal between 2 and 2:30 p.m. (可能性较上例小)
(22)If the ship were to leave at noon, it would pass through the canal between 2 and 2:30 p.m. (可能性最小)
[注]虚拟过去式有时并不表示“不大可能实现的假设”，而是表示一种希望或不希望发生的动作。如：
①If we caught the early train, we'd get there by lunch time. 假如我们赶上早班火车，到午饭时间我们就会到达那里了。(表希望)
②If we missed the train, we should have to wait an hour at the station. 假如我们赶不上这班火车，我们就得在车站等一小时。(表不希望)
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