на plus Accusative means motion on.
на plus Prepositional means location on.
English sometimes distinguishes motion ('to') vs. location ('at'), and sometimes doesn't: we use the same word 'on' with both motion ('Put it on the table') and with location ('It is lying on the table').
Russian distinguishes the two meanings by using different cases: Положќ её на стћл (Accusative) as against Онђ лежќт на столљ (Prepositional). Adverbs also make this motion/location distinction:
Кудђ тІ положќла кнќгу? Where did you put the book?
Гдљ кнќга? Where's the book?