Translation:The restaurant is open.
Is there any difference between offen and geöffnet?
Offen = open
Geöffnet = opened
But more common to see 'geöffnet' on a shop door in Germany.
Can be both
So perhaps 'ist offen' = 'is open' and 'hat geöffnet' = 'has opened'
So "offen" is adjective form of the verb "öffnen" - is that right?
Why not öffnen?
Öffnen is the verb, 'to open'.