If I had V-ed, I would have ...
结构︰If…had 过去分词…，主词 would/should 等 have 过去分词
说明︰这是与过去事实相反的假设<句型>。if <子句>要用<过去完成式>，<主要子句>则要用过去式<助动词>(would，should，ought to，might，could) have 过去<分词>。if <子句>中的 had可置于<主词>前面，而省略 If。
If I had seen him, I would have told him the truth.
I could have finished the task if I had had more time.
If we had known of your departure, we would have seen you off at the airport.
I someone had been with him, he might not have been drowned.
I might have been incredulous had I not been accustomed to such responses.
Had he not apologize to her, she would not have forgiven him.
Had he not saved me, I might have been drowned.
Had I proposed to you, would you have married me?
Had you not helped us, we should have canceled this transaction.