※ 引述《harry1234585 (UglyLoser)》之铭言：
: 课本上的句子：
: Many people might have died if there was a disaster.
: 疑问：
: 与过去事实相反的假设,if 子句不是要用过去完成式吗：Many people might have died
: if there had been a disaster.
: 求教是我理解有问题还是课本有谬误
这个句型，在板上好像讨论蛮多次了。之前经济学人那篇，也有用过去完成式，假定某件
事成真。
如果要用过去完成式，那时态就要设定在过去式。这可以画时间轴就知道了，假定A那件事
情发生成真，那B这件事就会怎样。如果一个用现在完成式(might have)，一个用过去完成
式就说不过去了(had been)。
要改写成过去完成式的话，就要这样子写：
Many people would die if there had been a disaster. （经济学人那句也是
had .... + would，两者都是在过去时间轴假设。）
经济学人原句，提供参考
"After Britain had left the eu early next year, the hard work of negotiating a
trade agreement would begin."
另外我特别查了Oxford Modern English Grammar，相关用法。
但实际上查到资料，发现使用更弹性，并没有一定要过去完成式这种说法，甚至不想要用
if 都没关系。
9.2.2.3 The modal past tense. (Page. 250)
The past tense of verbs can be used to talk about modal situations, for example
situations that are hypothetical or non-factual. For this reason we call it the
modal past tense, that is, conditions which are most likely to be fulfilled.
They can be introduced by if(only), imagine if, or suppose/supposing.
例句:
If you turned up for a day's shooting, the pheasants would die laughing.
If I was to go to Dubai, I can get things like electrical goods.
If only he had a darling, cuddly, relaxing dog, perhaps he wouldn't be such a
miserable toad.
不用if
Supposing he had the management team who so intelligently advise Beckham; might
it have made difference ?
也可以用It's time or wish
例句:
It's time people knew how their money was being spent. (事实上人们还不知道)
I wish it was over now. (但与现实相反)
这句也可以写成
I wish it were over now. 但were表示subjunctive verb form (西班牙文有，但是英文
没有，不过Be动词是例外)，用来表示a particular type of modal situation，来表示强
烈希望这件事情发生。
P.278
例如:
If I were you, I'd happy for the York position just for the experience.
你甚至可以用现在式，表示真的有可能完成
We can turn the if-clause in an example like into a clause expressing an open
condition, that is, a condition that can realistically be fulfilled, by
changing the past tense forms to present tense forms in both clauses.
例句:
If you turn up for a day's shooting, the pheasants will die laughing.
如果你学的只有过去完成式的用法，我建议可以换一本了。
不妨参考一下这本
Oxford Modern English Grammar
可以在Amazon 或 Kobo买到，现在Amazon kindle version 还有特价。