Ans：
假设法的用法：
1、假设的部分我们可以称之为条件子句
与事实相反的句子是由时态往前推一级,
如:条件子句而言,反现在用过去式,反过去用过去完成式,
与现在事实相反里的Be动词只用were
因此，若相反于过去事实，则应用过去完成式：
If＋S＋had＋V-en, S＋should (would, could, might)＋have V-en....
＝Had＋S＋V-en S＋should (would, could, might)＋have V-en....
例：如果当时我知道你要来，我会做个蛋糕给你。
If I had known you were coming, I should have baked a cake.
＝Had I known you were coming, I should have baked a cake.
2、遇到主要子句和条件子句所反的事实是属于不同时态时，必须分别依所反的时态来写。
因此，如果条件句相反于过去，而结果子句（主要子句）相反于现在，则改用：
If＋S＋had＋V-en, S＋should (would, could, might)＋V…now (today, this year, this time).
＝Had＋S＋V-en, S＋should (would, could, might)＋V…now (today, this year, this time).
例：假使你去年更用功研读英文的话，你现在或许就熟练它了。
If you had studied English more industriously last year, you might be proficient in it now.
＝Had you studied English more industriously last year, you might be proficient it now.
由上可知您问的
○ If it had rained a lot last month, we would have enough water now.
× If it had rained a lot last month, we would have had enough water now.
以上参自：
http://www.wretch.cc/bl.../13808669 http://www.nani.com.tw/nani/.../sleng_d.jsp (ps:以上解释若有错误，烦请高手不吝指正，谢谢。) 
