If my brother had been here last Sunday,he ________ me from going.(Unit8-13)

问题描述:

If my brother had been here last Sunday,he ________ me from going.(Unit8-13)
为什么用would have prevented不用would prevent?请分析一下句型或语法.

这是一个虚拟语气的用法
与现在事实相反 动词的过去式(be的过去式一般用were) would/ should/ could/ might + 动词原形
与过去事实相反 had + 过去分词 would/ should/ could/ might + have + 过去分词
与将来事实相反 动词过去式,should + 动词原形,were to + 动词原形 would/ should/