는데 모르다

…, but doesn't know; you never know

-는데 sets up a contrast and 모르다 supplies the second half. Straight contrast: 나 나름 안다고 생각했는데 몰랐네 'I thought I knew, but I didn't', 먹을 게 옆에 있는데 모를 수가 있지? 'how can you not notice food right beside you?'. It also hedges what you have just said: 트윈이라고 얘기는 했는데 모르죠 'he said they were twins, but you never know'.

Register: casual