Question

that what which whom who在宾语从句中的用法

Answer 1

引导宾语从句的that之后接的从句不长时，可以省略。如果that后面的宾语从句较长时，that不能省略。
I hope (that) you'll be fine soon. 我希望你尽快康复。
名词从句中that,what用法比较：
引导名词从句的that是连词，在句中无成份，无意义，而what是连接代词，what＝the
thing(s) that.
例如：It was told in yesterday´s newspaper that what the students had done was praised
by the factory.
连接代词who，whom ，whose，what， which：它们起连接作用，作句子成分，各有其自己的意义。例如：
6）The teacher asked the new student which class he was in？（武汉市）（which引导宾语从句，在从句中作定语，修饰class，意为“哪个”。）
7）Can you tell me whom he is waiting for？（天津市）
望楼主满意我的回答，如有疑问，在线商榷

Answer 2

that what 在名词性从句中成分主要区别就是：that不做任何成分，what充当成分
which 定语
whom宾
who主语