Had I worn my overcoat,I ________ have caught cold.(Unit7-12)

题目

Had I worn my overcoat,I ________ have caught cold.(Unit7-12)
为什么用couldn't不用won't?请分析一下句型或语法.
那这里用了特殊句型吗？换成正常语序并补全该句，应是怎样的呢？

答案

这是句虚拟语气的if倒装句,后半句是主句,陈述的是对过去已经发生了的事情的另一种假设,所以用过去式
正常语序：If I had worn my overcoat,I couldn't (这里也可以用wouldn't) have caught cold.