Сљсть/садќться and лљчь/ложќться imply motion; therefore, the Accusative case (as in за стћл) is used after the relevant prepositions and the question-word кудђ is used (not гдљ) Compare:
— Кудђ ћн сљл?
— На дивђн.
"Where did he sit down?"
"On the couch."
— Гдљ ћн сидљл?
— На дивђне.
"Where was he sitting?"
"On the couch."
Ћн лёг на дивђн. He lied down on a couch.
Ћн лежђл на дивђне. He was lying on a couch.