为什么有的宾语从句引导词只能用that

宾语从句通常有两种,1）动词+宾语从句；2）形容词+宾语从句一般来说,第二类宾语从句连词基本为that,如：be sure that。。。学习第一类宾语从句时,通常老师都会让学生练习直接引语变间接引语。当直接引语是陈述句时,间接引语的连词为that。例如：直接引语：Tom said," I will read it。"间接引语：Tom said that he would read it。