That's why..

宾语语从句大体分为四种类型：
一、that 引导的陈述具体事实的从句,如：
He said that he had been there before。(陈述了过去曾经去过那个地方)
二、wh- 疑问词引导的提出对具体问题陈述的从句,包括代词 what who,which 和副词 when,where,why。如：
I don't know where he is now。(提出对此刻在什么地方的疑问)
I wonder who he is,where he came from and why he came。我很想知道他是谁,从哪里来,来干什么。（提出具体问题）
三、if/whether 引导提出或然性问题（是否）陈述的从句,如：
They didn't tell us if/whether they would com。
四、关系代词性从句,即引导词可拆分为先行词 + 关系代词,相当于汉语的所字结构,即“所。。。的”；如：
That's why I didn't see the sign。那就是我所没有看到信号的原因。(why = the reason that)
This is what he will do。这就是他们所干的（事情）(what = anything that)