that在名词性从句里都是不充当成分的,没有实际意义,包括宾语从句,主语从句,同位语从句,表语从句,并且在宾语从句里经常省略,所以,不能用that做主语.另外,that在定语从句中是做成分的,可以做主语.
不一定要用it,根据句子需要
例如：He admitted that he would be our new manager.he是主语,that不做成分,无意义
He admitted that it was difficult to work with Tom.it 做形式主语