与虚拟语气无关.主将从现只用于时间状语从句和条件句.
1.时间状语从句：明天他来时,我会告诉他的.I will tell him when he comes tomorrow.
2.条件句：如果明天下雨,我们就呆在家里.If it rains tomorrow,we will stay at home.
虚拟语气属于条件句.而真实的条件句都必须用陈述语气.（比如上一句）
虚拟语气基本上分三种时间段：
1 对过去发生完的事情,埋怨,或猜测.如：如果你不去那里的话,就不会发生这事情了.
If you had not gone there,it would not have happened.
2.对现在的推测,如：如果我是你的话,我就去那里.If I were you,I would go there.
3.对未来推测,如：如果明天地球没有水的话,我们怎么办?What should we do if there should no water on the earth tomorrow?