你可以参照这样一个公式：
对现在虚拟：从句（If+主语+动词过去式）, 主句（主语+should/would+动词原形）
对过去虚拟：从句（If+主语+had+过去分词）, 主句（主语+should/would+have+过去分词）
对未来虚拟：等同对现在虚拟
分别给例子如下：
If I were you, I would not let her go.
If I had met you back in high school, I would not have chosen this chollege.
If you lived here for a while, you would change your mind.
ps：英式英语中主语里面,I 用should,其它用would；美语不管的,统统用would.