虛擬語氣很簡單，首先大多是主將從現的句子，然後主從都虛擬的話，全部改成過去。
虛擬，就是與事實相反，或發生的可能性極小。
從句對過去的虛擬用had done，對現在的虛擬用過去式（be用were），對將來的虛擬用過去式/should（should可省略）/were to+v.;
主句對過去虛擬用should/would/could/might+have done, 對現在虛擬用should/would/could/might+v., 對將來虛擬用should/would/could/might+v.
Had it not been for you, I would not have finished my work. (主從句都是對過去的虛擬）。
If he were here, he would enjoy the film. (主從句都是對現在的虛擬）
If it were to rain tomorrow, we would stay at home. (主從句都是對將來的虛擬）
主句和從句所虛擬的時間有時候可以互換，比如從句是對過去的虛擬，主句是對將來的虛擬。如：
If they had left home early, they would arrive in half an hour.（實際上他們也沒有早離家，當然也不可能在半個小時內到。）
有些詞常用虛擬, 如
for fear that, in case, in order that, lest, so that
那天讀《福爾摩斯之四簽名》看到一句：
If the country should be at peace, the treasure will be divided among them.
這句話的條件是虛擬，但是主句是真實句。這種句子的條件是表示有很小的可能性實現。這句話的背景條件是整個印度一片混亂，當時他們也不知道國家會不會太平，但他們4個人要瓜分財寶，這肯定是真的。
虛擬語氣還有很多細緻的內容，建議找本語法書學習下，然後從閱讀中去熟悉。
比如上面那句四簽名中的句子，就是我初一的兒子讀到的時候不明白問我的。
比如The hunger games中就有很多虛擬語氣的句子。當然，每本原著中幾乎都有這樣的句子。比如我兒子讀的第一套章節書《瘋狂學校》（My weird school）中就有I wish I were in your shoes這樣的虛擬句。
看看你說的這句話：
（Only were it to snow）exceptionally heavily might railway services be temporarily dislocated.
這句話主從句都是對將來的虛擬。說明這種惡劣天氣是極少極少發生的，火車執行也極少可能被打亂。所以用了虛擬來強調這種非常少的可能性。使用虛擬語氣，我們就可以看出，這種情況過去沒有發生過，將來發生的可能性也是非常小的。
如果使用真實條件句，那說明火車執行曾經被打斷過，而且不止一次，並且是常規性的，一碰到大雪火車就停開。
而我們文中說了，飛機航班可能因惡劣天氣而取消輪船也可能因風暴而推延，都是用了may, 說明這種可能性是經常發生真實存在的。而火車因exceptionally heavy snow而晚點或者是停開這種情況幾乎是不可能發生的，所以用了虛擬。
《新概念英語》第二冊的虛擬語氣句子：
L38 Everything except the weather
He acted as if he had never lived in England before. （as if 引導虛擬條件句，並且是對過去的虛擬，所以用had lived。而實際情況是，他過去曾經住在英國，因為第1句就提到他回到英國。）
L40 Food and talk
If you ate more and talked less, we would both enjoy our dinner.（主從句都是對現在的虛擬，實際上作者一直在試圖搭訕）
本課後面的練習（以下主從句均是對現在/將來的虛擬）：
He would enjoy this if he were present.
If you broke this window, you would have to pay for it.
If he were clever, he would not have any difficulty.
What would you do if you won a lot of money?
If I were you I wouldn"t be so confident.
If you could help me, I would be grateful.
L64 The channel tunnel
If, at the time, the British had not feared invasion, it would have been completed. （主從句都是對過去的虛擬）
本課後面的練習：
If you had told me about it earlier, I wouldhave been able to help you.
If you could have come with us, we would have been pleased.
You would not have made such a mistake if you had been more careful.
If father were alive, he would be horrified.
L71 A famous clock
If the Houses of Parliament had not been burned down in 1834, the great clock would never have been erected.（主從句都是對過去的虛擬）
L88 Trapped in a mine
If they knew how difficult it was to drill through the hard rock, they would lose heart.（主從句都是對現在的虛擬）
L95 A fantasy
If I had been, I would not have been able to get home for lunch.（主從句都是對過去的虛擬）
虛擬語氣很簡單，首先大多是主將從現的句子，然後主從都虛擬的話，全部改成過去。
虛擬，就是與事實相反，或發生的可能性極小。
從句對過去的虛擬用had done，對現在的虛擬用過去式（be用were），對將來的虛擬用過去式/should（should可省略）/were to+v.;
主句對過去虛擬用should/would/could/might+have done, 對現在虛擬用should/would/could/might+v., 對將來虛擬用should/would/could/might+v.
Had it not been for you, I would not have finished my work. (主從句都是對過去的虛擬）。
If he were here, he would enjoy the film. (主從句都是對現在的虛擬）
If it were to rain tomorrow, we would stay at home. (主從句都是對將來的虛擬）
主句和從句所虛擬的時間有時候可以互換，比如從句是對過去的虛擬，主句是對將來的虛擬。如：
If they had left home early, they would arrive in half an hour.（實際上他們也沒有早離家，當然也不可能在半個小時內到。）
有些詞常用虛擬, 如
for fear that, in case, in order that, lest, so that
那天讀《福爾摩斯之四簽名》看到一句：
If the country should be at peace, the treasure will be divided among them.
這句話的條件是虛擬，但是主句是真實句。這種句子的條件是表示有很小的可能性實現。這句話的背景條件是整個印度一片混亂，當時他們也不知道國家會不會太平，但他們4個人要瓜分財寶，這肯定是真的。
虛擬語氣還有很多細緻的內容，建議找本語法書學習下，然後從閱讀中去熟悉。
比如上面那句四簽名中的句子，就是我初一的兒子讀到的時候不明白問我的。
比如The hunger games中就有很多虛擬語氣的句子。當然，每本原著中幾乎都有這樣的句子。比如我兒子讀的第一套章節書《瘋狂學校》（My weird school）中就有I wish I were in your shoes這樣的虛擬句。
看看你說的這句話：
（Only were it to snow）exceptionally heavily might railway services be temporarily dislocated.
這句話主從句都是對將來的虛擬。說明這種惡劣天氣是極少極少發生的，火車執行也極少可能被打亂。所以用了虛擬來強調這種非常少的可能性。使用虛擬語氣，我們就可以看出，這種情況過去沒有發生過，將來發生的可能性也是非常小的。
如果使用真實條件句，那說明火車執行曾經被打斷過，而且不止一次，並且是常規性的，一碰到大雪火車就停開。
而我們文中說了，飛機航班可能因惡劣天氣而取消輪船也可能因風暴而推延，都是用了may, 說明這種可能性是經常發生真實存在的。而火車因exceptionally heavy snow而晚點或者是停開這種情況幾乎是不可能發生的，所以用了虛擬。
《新概念英語》第二冊的虛擬語氣句子：
L38 Everything except the weather
He acted as if he had never lived in England before. （as if 引導虛擬條件句，並且是對過去的虛擬，所以用had lived。而實際情況是，他過去曾經住在英國，因為第1句就提到他回到英國。）
L40 Food and talk
If you ate more and talked less, we would both enjoy our dinner.（主從句都是對現在的虛擬，實際上作者一直在試圖搭訕）
本課後面的練習（以下主從句均是對現在/將來的虛擬）：
He would enjoy this if he were present.
If you broke this window, you would have to pay for it.
If he were clever, he would not have any difficulty.
What would you do if you won a lot of money?
If I were you I wouldn"t be so confident.
If you could help me, I would be grateful.
L64 The channel tunnel
If, at the time, the British had not feared invasion, it would have been completed. （主從句都是對過去的虛擬）
本課後面的練習：
If you had told me about it earlier, I wouldhave been able to help you.
If you could have come with us, we would have been pleased.
You would not have made such a mistake if you had been more careful.
If father were alive, he would be horrified.
L71 A famous clock
If the Houses of Parliament had not been burned down in 1834, the great clock would never have been erected.（主從句都是對過去的虛擬）
L88 Trapped in a mine
If they knew how difficult it was to drill through the hard rock, they would lose heart.（主從句都是對現在的虛擬）
L95 A fantasy
If I had been, I would not have been able to get home for lunch.（主從句都是對過去的虛擬）