When to use ”that” ?

Hello :)

So, I've recently started reading in English, and in one of my books there is this sentence: "Dear, mom, I know you know ..."

Personally, I would have said "I know you know". Now (that ?) I know (that ?) I was wrong, I wonder if there is a way to know when to use "that" in this kinds of situations.

Sorry if i haven't been clear enough..

I'll give you some examples if you don't understand me :)

Thanks for helping me :)