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.