定语从句引导词that、which 在从句中作宾语、表语时可以省略.

I don't like this lilm,which isn't interesting at all.这是which引导的非限制性定语从句,不能省略which.
He said that he had been to Beijjing.这是that引导的宾语从句,that在从句中不担当成分,可以省略.
It is a fact that he likes watching TV.这是that引导的同位语从句,that不能省略.

I know the answer, which is sure. (which引导非限制性定语从句是不能省略）
I know (that )I know the answer. (that 可省略）
My answer is that I know it. (引导同位语从句时不能省略。)

that：John said that he liked it very much

Tom was late that surprised me. 中的that 可省略 Tom was late, which surprised me.中的which不可省略。