All that"s important is that you are doing your best and moving in the right direction 为什么要填ALL

题目

All that"s important is that you are doing your best and moving in the right direction 为什么要填ALL
答案说All作that"s important 定语从句的先行词（为什么要用ALL作先行词,that"s important 不能作先行词,怎样才为先行词?）.All that相当于what引导的名词性从句（为什么说下定语从句,一会又说是名词性从句,难不成这句话可理解为名词性从句,）.让我感到很乱,难以理解,括号里的就是我的疑问,或者有更好的解释也可以,我基础比较薄弱,最好明了一些,

答案

all 是先行词,that's important 是定语从句,对其修饰限制.
在语法上,all that =what,所以,由定语从句知识可以跨越到名词性从句,即：what is important is that .
不必担心,背下来,见多了,就见怪不怪了.