與現(xiàn)在事實(shí)相反 If I (we, you, he, they) +動(dòng)詞過去式(be的過去式用were) I (we) should/you would/he would/they would+動(dòng)詞原形
與過去事實(shí)相反 If I (we, you, he, they) + had+過去分詞 I (we) should/you would/he would/they would+ have+過去分詞
與將來事實(shí)可能相反 If I (we, you, he, they) +動(dòng)詞過去式(be的過去式用were) I (we) should/you would/he would/they would+動(dòng)詞原形
[注]美國英語的結(jié)果主句，不管什么人稱，皆常用would.
1)與現(xiàn)在事實(shí)相反的非真實(shí)條件句，條件從句的謂語用動(dòng)詞的過去式(be的過去式用were)，結(jié)果主句的謂語用should(第一人稱)或would(第二、三人稱)+動(dòng)詞原形。如：
(3)If we left now, we should arrive in good time. 假如我們現(xiàn)在就走的話，我們就會(huì)及時(shí)到達(dá)。
(4)If I were you, I would refuse the money. 假如我是你的話，我就不會(huì)要那錢。(would代替了should)
(5)Even if he had the money, he wouldn't buy it. 他即使有錢也不會(huì)買它。
2)與過去事實(shí)相反的非真實(shí)條件句，條件從句的謂語用had+過去分詞，結(jié)果主句的謂語用should(第一人稱)或would(第二、三人稱)+ have+過去分詞。如：
(6)We would've dropped by if we had had time. 假若我們有時(shí)間的話，我們就會(huì)順道拜訪了。
(7)I wouldn't have known what these were for is I hadn't been told. 假如別人不告訴我，我就不會(huì)知道這些東西是干什么的了。
但虛擬式were也可表與過去事實(shí)相反。如：
(8)If I were not busy, I would have come. 假如我不忙，我就會(huì)來了。
表過去時(shí)間的假設(shè)亦可用虛擬式should have+過去分詞，這種虛擬式所表的動(dòng)作并非完全不可能發(fā)生，它只是表示一種偶然性，并常和直陳語氣連用。但也可與虛擬語氣連用。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，她的遭遇會(huì)如何。
美國大眾語常用would have+過去分詞表示與過去事實(shí)相反的假設(shè)。如：
(10)If this sleeplessness would have been allowed to go on, she would have collapsed. 這種失眠如果任其發(fā)展，她就會(huì)垮掉的。
3)與將來事實(shí)可能相反的非真實(shí)條件句，條件從句的謂語用動(dòng)詞的過去式(be的過去式用were)，結(jié)果主句的謂語用should(第一人稱)或would(第二、三人稱)+動(dòng)詞原形。如：
(11)If you dropped the glass, it would break. 假如你把玻璃杯掉在地上，它會(huì)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時(shí)間，你就會(huì)改變對(duì)那地方的看法了。
[注]如無上下文，下面句中的rained有歧義，可能是表與將來事實(shí)可能相反的虛擬語氣，也可能是表過去習(xí)慣的直陳語氣：If it rained we would stay indoors and read. 如果下雨，我們就待在家里看書。
在與將來事實(shí)相反的條件句中，其條件從句的謂語亦可用were to+動(dòng)詞原形。這種虛擬時(shí)式比較正式，多用于書面語中，其假想性很強(qiáng)，實(shí)現(xiàn)性甚小。如：
(13)If he were to come, what should we say to him? 假如他來了，我們對(duì)他說什么呢?
在與將來事實(shí)相反的條件句中，其條件從句的謂語還可以用should+動(dòng)詞原形。這種形式并不強(qiáng)調(diào)意愿，只強(qiáng)調(diào)一種有偶然實(shí)現(xiàn)的可能性。如：
(14)If he should see me, he would know me. 假如他看見我，就會(huì)認(rèn)識(shí)我。
這種形式往往有不為人所歡迎的含義。如：
(15)If he should go away, I should be grieved. 他如離去，我就會(huì)感到悲傷。
有時(shí)可用would代替should，以免與結(jié)果主句中的should重復(fù)。如：
(16)If you would be interested, I should be very glad to send you a copy of my book. 你如感興趣，我將高興寄給你一冊(cè)我的書。
在這種條件句中，結(jié)果主句常用直陳語氣或祈使語氣。如：
(17)If the train should be late, what will you do? 如果火車晚點(diǎn)了，你將怎么辦?(結(jié)果主句用疑問句)
(18)If we should fall in this, we are ruined. 我們這個(gè)計(jì)劃如果失敗，我們就倒霉了。(結(jié)果主句用直陳語氣)
(19)If you should meet Henry, tell him I want to see him. 假如你見著亨利，告訴他我要見他。(結(jié)果主句用祈使語氣)
關(guān)于過去式、should+動(dòng)詞原形與were to+動(dòng)詞原形這三種虛擬式的可能性孰大孰小的問題，語法家們認(rèn)為它們的可能性雖都不大，但相比較，were to+動(dòng)詞原形的可能性最小，should+動(dòng)詞原形次之，過去式更次之。試比較：
(20)If the ship left at noon, it would pass through the canal between 2 and 2:30 p.m. 這艘船如果中午啟航，下午2時(shí)至2時(shí)半之間將穿過運(yùn)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. (可能性較上例小)
(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]虛擬過去式有時(shí)并不表示“不大可能實(shí)現(xiàn)的假設(shè)”，而是表示一種希望或不希望發(fā)生的動(dòng)作。如：
①If we caught the early train, we'd get there by lunch time. 假如我們趕上早班火車，到午飯時(shí)間我們就會(huì)到達(dá)那里了。(表希望)
②If we missed the train, we should have to wait an hour at the station. 假如我們趕不上這班火車，我們就得在車站等一小時(shí)。(表不希望)
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