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.

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