If I had V-ed, I would have ...
结构︰Ifhad+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 +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.
如果我当时看见他，我会告诉他实情。
I could have finished the task if I had had more time.
如果当时我能有多一点时间，我就能完成这项任务。
If we had known of your departure, we would have seen you off at the airport.
如果我们事先知道你要离开，我们会到机场送行。
I someone had been with him, he might not have been drowned.
如果有人在他旁边，或许他就不会溺死。
I might have been incredulous had I not been accustomed to such responses.
若非我已经习惯了这样的回答，我可能不敢轻易相信。
Had he not apologize to her, she would not have forgiven him.
她不会原谅他，若非他道了歉。
Had he not saved me, I might have been drowned.
我可能溺死了，若非他救了我。
Had I proposed to you, would you have married me?
我要是当年向你求婚，你会答应嫁给我吗?
Had you not helped us, we should have canceled this transaction.
假如没有你的帮助，这笔生意早就告吹了。