στη / στην
while repeating the places lesson today I was asked to translate "he is in prison", and I wrote "αυτός είναι στην φυλακή" This was considered "almost correct", the fully correct one being written with στη. My Constantinopolian Greek friend used to say it is okay to use στη with some words but it would be correct to use στηv with all words. Is this not relevant with Greek spoken in Greece? Or have I got it wrong?