虚拟语气很简单，首先大多是主将从现的句子，然后主从都虚拟的话，全部改成过去。
虚拟，就是与事实相反，或发生的可能性极小。
从句对过去的虚拟用had done，对现在的虚拟用过去式（be用were），对将来的虚拟用过去式/should（should可省略）/were to+v.;
主句对过去虚拟用should/would/could/might+have done, 对现在虚拟用should/would/could/might+v., 对将来虚拟用should/would/could/might+v.
Had it not been for you, I would not have finished my work. (主从句都是对过去的虚拟）。
If he were here, he would enjoy the film. (主从句都是对现在的虚拟）
If it were to rain tomorrow, we would stay at home. (主从句都是对将来的虚拟）
主句和从句所虚拟的时间有时候可以互换，比如从句是对过去的虚拟，主句是对将来的虚拟。如：
If they had left home early, they would arrive in half an hour.（实际上他们也没有早离家，当然也不可能在半个小时内到。）
有些词常用虚拟, 如
for fear that, in case, in order that, lest, so that
那天读《福尔摩斯之四签名》看到一句：
If the country should be at peace, the treasure will be divided among them.
这句话的条件是虚拟，但是主句是真实句。这种句子的条件是表示有很小的可能性实现。这句话的背景条件是整个印度一片混乱，当时他们也不知道国家会不会太平，但他们4个人要瓜分财宝，这肯定是真的。
虚拟语气还有很多细致的内容，建议找本语法书学习下，然后从阅读中去熟悉。
比如上面那句四签名中的句子，就是我初一的儿子读到的时候不明白问我的。
比如The hunger games中就有很多虚拟语气的句子。当然，每本原著中几乎都有这样的句子。比如我儿子读的第一套章节书《疯狂学校》（My weird school）中就有I wish I were in your shoes这样的虚拟句。
看看你说的这句话：
（Only were it to snow）exceptionally heavily might railway services be temporarily dislocated.
这句话主从句都是对将来的虚拟。说明这种恶劣天气是极少极少发生的，火车运行也极少可能被打乱。所以用了虚拟来强调这种非常少的可能性。使用虚拟语气，我们就可以看出，这种情况过去没有发生过，将来发生的可能性也是非常小的。
如果使用真实条件句，那说明火车运行曾经被打断过，而且不止一次，并且是常规性的，一碰到大雪火车就停开。
而我们文中说了，飞机航班可能因恶劣天气而取消轮船也可能因风暴而推延，都是用了may, 说明这种可能性是经常发生真实存在的。而火车因exceptionally heavy snow而晚点或者是停开这种情况几乎是不可能发生的，所以用了虚拟。
《新概念英语》第二册的虚拟语气句子：
L38 Everything except the weather
He acted as if he had never lived in England before. （as if 引导虚拟条件句，并且是对过去的虚拟，所以用had lived。而实际情况是，他过去曾经住在英国，因为第1句就提到他回到英国。）
L40 Food and talk
If you ate more and talked less, we would both enjoy our dinner.（主从句都是对现在的虚拟，实际上作者一直在试图搭讪）
本课后面的练习（以下主从句均是对现在/将来的虚拟）：
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. （主从句都是对过去的虚拟）
本课后面的练习：
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.（主从句都是对过去的虚拟）
L88 Trapped in a mine
If they knew how difficult it was to drill through the hard rock, they would lose heart.（主从句都是对现在的虚拟）
L95 A fantasy
If I had been, I would not have been able to get home for lunch.（主从句都是对过去的虚拟）