bunu bilmiyordum

This is one of the common mistakes that people do while learning Turkish Language. When you did not know something, you do not use the past simple tense in Turkish-which is I did not know this-Bunu bilmedim. We do not say "Bunu bilmedim". But we use the past imperfect instead and say; 

"Bunu bilmiyordum"

Bunu biliyor muydunuz? Did you know this?

Evet, biliyordum. Yes, I knew.

Hayır, bilmiyordum. No, I didn't.