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
Adding to the other comment, there’s a reasonably well known book series called:
Der Dativ ist dem Genetiv sein Tod. [The dative is the death of the genitive].
Language changes and the genitive is slowly dying out. Instead of saying “das Bein des Stuhls” you’d say “das Bein vom Stuhl” or perhaps simply “das Stuhlbein”.
There’s also “des Stuhles Bein” as you mentioned but this’ll make you sound like you’re from the 19th century.