Re: по + accusative or dative in temporal expressions
With great humility and all due respect to the native speakers, по doesn't take the genitive. As Maroseika says in #5, in the phrase по два/три/четыре часа в неделю, по is governingthe numbers два, три and четыре, which are in the accusative case, and the noun часа that follows these numbers is required to be in the genitive singular. The noun case here is governed by the numerals два, три or четыре, not by the preposition по.
(Ин май хамбл апиньон, конечно)
Two (hundred) countries separated by a common language ...