他们每天在写日记，这会让他们表达更加流利和雄辩，距掌握语言更近了一步。
点睛本句的主干是that puts theme。every day为时间状语；that引导的是主语从句；a step closer to fluency, eloquence, and command of language为宾语them的补语。
command作名词与介词of连用意为“使用或控制某事物，掌握”。如：
He has a good command of English. 他精通英语。
考点归纳常见的主语从句有以下几种：
由that引导。如：
That he hasn't called is odd. 他没来电话真是很奇怪。
It's certain that prices will go up. 物价要上涨是肯定的。
由连接代词或连接副词引导。如：
Why they left wasn't important. 他们为什么离开并不重要。
It was uncertain whether she would come or not. 她来不来还不确定。
由关系代词what或whatever引导。如：
What I want is a watch. 我想要的是一块手表。
Whatever you did was right. 你做的一切都是对的。