if引导的虚拟条件句分为虚拟条件句和真实条件句
前者时态要推前一个时态
如：If I were you,I would go(对现在的虚拟,推前一个时态)
后者遵循“主将从现”（即主句用将来时,从句用现在时）
如：If it rains tomorrow,I will stay at home.
if引导宾语从句不能放在介词后