"The book is by the newspaper."
Translation:Το βιβλίο είναι δίπλα στην εφημερίδα.
Can someone explain why στην is proper here? It's because δίπλα στην is a phrase, right? and not because εφημερίδα is in the accusative case? I don't see how εφημερίδα is rightly in the accusative case here, if that is so. If anyone could explain.
mizinamo gives what I think is a very good explanation in this thread: https://www.duolingo.com/comment/19132678 (also from this section)
«δίπλα» is an adverb and thus needs the preposition «σε» to lead into the noun «την εφημερίδα». The article «την» is accusative because «εφημερίδα» is accusative (and «εφημερίδα» is accusative because it is in a prepositional phrase). «σε» and «την» contract to form «στην». A more direct translation of «δίπλα σε/δίπλα στην» would be "next to/next to the".
So to answer your second question, «δίπλα σε» is a regularly occurring phrase. In this case «σε» turns into «στην» because the noun phrase is «την εφημερίδα», but you could also have sentences like «το βιβλίο είναι δίπλα σε μία εφημερίδα» (the book is next to a newspaper) and «αυτός στέκεται δίπλα σε κάποιον» (he stands next to someone)
Since «είναι» is a linking verb, «εφημερίδα» could be in the nominative case as a predicative complement but only if it wasn't part of a preposition, e.g. «το βιβλίο δεν είναι η εφημερίδα» (the book is not the newspaper)