总的来说,if 引导的从句有三种用法
1.用于真实条件句
例如：
If you are at home,I will go to see you.
If you work hard at it,you can get success.
表示,只要按这样得条件去做,结果是肯定的.
2.用于虚拟条件句
例如：
If I were you,I wouldn't go there.
If the sun rose from the west,I would call you Dad.
表示,条件是假设的,不真实的,因此,其结果也是不可能实现的.
注意句中谓语有所变化
3.用于名词性从句,意思等于whether or not 表是否之意.
例如：
It doesn't matter if you go to attend the meeting.
你是否参加会议没关系.