Tá an fhuinneog ar oscailt
The window is open

an bhfuil sí dúnta
Is it shut?

Can somebody explain to me the difference? Arn't oscailt and dúnta the same "form" of different verbs? (I think they are the verbal noun here....)

No. oscailt is the verbal noun of oscail, and dúnta is the verbal adjective of dún.