I am certain that he will deliver on his promise 这个that是什么从句?

问题描述:

I am certain that he will deliver on his promise 这个that是什么从句?

I am certain that he will deliver on his promise
that 是宾语从句为什么?怎么是主语从句?不是宾从或者其他的呢?怎么看出来的是宾语从句这里的that不可省略,在句子中充当了一定的成分I am certain that that在句子中做宾语that就指的是 he will deliver on his promise