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?
Officially it's not correct. Real-life though, what your friend said is perfectly correct.
Thank you again! Is it like it's okay to speak it but you just don't write it that way?
Exactly, you can speak it either way, but write it with -ν only if it is followed by a vowel or κ,π,τ,ψ,ξ,γκ,γγ,μπ,ντ. Usually in verbal speech it comes naturally to follow the rule, but it's fine not to do so.