Sit
According to different kinds of seat, the verb ‘sit’ may be followed by different prepositions.
Incorrect : He sat on his desk to work.
Correct : He sat at his desk to work.
Incorrect : She sat down over the sofa/chair.
Correct : She sat down on the sofa/chair.
Incorrect : He was sitting on an arm-chair.
Correct : He was sitting in an arm-chair.