我觉得这里有个中国学生不太熟的陷阱，似乎国内语法书没有讲过。就是If I were you的情形。我以If I were you, I would go. 为例谈一下我的理解。If I were you可以用于错综时间，如：If I were you, I would have gone. （与过去事实相反）。其实很多Native speakers 这样用。 如果说If I had been you, I would have gone. 这样更符合语法，但一般日常不这么用。我想道理在于“我不是你”是永恒不变的事实，无论过去、现在还是将来。所以人们习惯上用If I were you,（即与现在这个事实相反的假设）来表达不同时间的假设。如果把you换成其他词就不一定行。例如：If I were rich, I would have helped you five years ago. 这种表述，很多母语的人会认为是错的。因为不合乎逻辑，要用If I had been rich. 因为人们是否rich 不是一生不变的，和If I were you 不同。 所以你提到的一本语法上写的If I were you, I wouldn't do it. 认为是与现在事实相反的虚拟语气,这是正确的。判断主句到底是与现在事实相反，还是与过去或将来事实相反，就根据虚拟语气的构成形式判断即可。有的时候需要看语境，如：If you didn’t come tonight, I would have to put off our appointment to a later date. 这的句子显然指将来。其实，是因为If I were you的特殊性，使你对虚拟语气不知所措了。你可以参考国外的一个网站谈到了If I were you的问题，也许对你有所帮助。https://gonaturalenglish.com/second-and-third-conditional/