首先没必要记你老师的退时态说法,记住规则就行,记住例句
与现在相反：从句用过去式、were 主句 would/should/could do if I were rich, I would buy BMW
与过去相反从句用过去完成时 主句用would/ should /couldhave done if i had followed your sugesstion, i would have passed the test.
与将来相反 从句用过去式 /were to do/should do 主句用 would/could /shoulddo if it were to rain tomorrow, we would stay inside.
当然还有混合虚拟,以及其他虚拟 最好看看语法书 很简单的 祝你成功