为什么有的句子要用that呢?有的从句就没有

that在引导从句时的省略现象
一、that引导定语从句时,充当关系代词.
A.that在定语从句中作宾语时,往往可省略.He is the man(that)I gave the money to.B.that在定语从句中作主语时,不可省略.He'is the man that lives next door.C.that在way,distance后面常可省略.I went out the same way(that)I'd got in.
二、that引导名词性从句时,充当连词,本身无实际意义.
A.that引导主语从句、同位语从句及表语从句时,正规语中,通常不可省略.It was known long ago that air has pressure.They all know the fact that Taiwan is part of China.Their suggestion is that should improve their working conditions.B.that在引导宾语从句时,通常可省略.We know(that)sound can travel through air.但：1）在介词后的宾语从句中that常不可省略,且前面往往有个形式宾主it:You may depend on it that they will support your plan.See to it that you are not late again.Except that he speaks too fast,he is an excellent teacher.2)两个宾语从句连用时,即使省略了第一个that,也不可省略第二个that.I believe(that)you've done your best and that things willimprove.
三、that引导状语从句时,充当连词,有时也可省略.
A（结果状语从句）The sound is so weak(that)you can't hear it.B(结果状语从句)Now(that)I have finished my lessons,I intend to go for a walk.C(宾主从句)We are delighted(that)you can come.D(某些边际连词)如：the monent that,the minute that,the instant中省略that.The monent(that)he spoke,we recognized his voice

That的从句都是可以省略that的