"He is in prison."
Translation:Είναι στη φυλακή.
should have been είναι στη φυλακή; it's changed now. Sometimes, native speakers omit articles when they talk about a place, so είμαι σπίτι=i am at home, but this is very colloquial.
Thanks! Yeah, I'm familiar with πάω σπίτι, but not with είναι φυλακή.. Perhaps someone wanted the variant (is it ok?) είναι ΣΕ φυλακή without the article?