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