As you know.
ご存じより
As per your understanding
I know him
よく知っていた
お分かり
Your understanding
That is the only thing that I know.
Rather than that I knew, perhaps I should say...
You might already know this, but. . .
You ought to know.
よく知ってるね
something that is familiar to someone
教えてくれ
As if you did not know!