I am sorry I didn't know that.请问这是从句吗?是什么从句?

是宾语从句
有些形容词具有动词的含义,所以也可以带一个宾语从句,例如；
I am sorry I am late.
I am glad that you can join us.
Are you sure his answer is right?
常用来引导宾语从句的形容词有：sure,certain,glad,please,happy,sorry,afraid,satisfied,surprised
I am sure I will pass the exam.
我确信我会通过考试.
I am sorry that I have troubled you so long.
很抱歉我这么长时间在打扰你.
He is glad that Li Ming went to see him when he was ill.
他很高兴在他生病的时候李明能去看望他.