what表示“所...”
eg:
That's what I wanted to say
that引导的从句说明的是具体的一件事.
eg:That he didn't go to school made his monther angry.
引导that 在引导主语从句,表语和同位语从句时不能省略,在引导宾语从句时可以省略.
eg:He said (that) he had been to shanghai twice.