[SOLVED] Difference between der/das/die and den

Both der/das/die and den mean 'the' but when are they supposed to be used. Are they interchangeable?

I recently came across a statement on duolingo : Mia, der Stuhl ist da druben, Warum brauchst du den Stuhl.

Since it is translated as 'the' in both cases why is Mia, den Stuhl ist da druben, Warum brauchst du der Stuhl wrong