与现在事实相反的虚拟，if从句用过去式，主句用主语+should / would / could / might，这是虚拟语气的时态倒退。比如：
If I were you, I would reconsider their advice.
假如这句话是事实的话是：If I am you, I will reconsider their advice.
以上这个我明白。
但为什么如果这句话是事实的话不能这样说：
If I am you, I would reconsider their advice?
would不是will更加委婉的用法吗？那这句话的时态倒退是怎样的，would本身是will的过去式，但它作为和will意思有差别的情态动词时有过去式吗？
