Sorry if I sound stupid just trying to understand as well as I can, but why does "eine Schlange hat den Frosch gefressen" take a "den" instead of "dem"? I thought den was only used for possession of plural objects and seeing as Frosch is a singular item this kind of confused me.
"Den" is used for plural objects in the dative case. This is in contrast to the masculine singular "dem."
"Den" is also used for masculine singular objects in the accusative case. The nominative is "Der Frosch" but since the frog is being eaten, it is the direct object in the accusative case.
You would use the genitive case (not the dative) for a possessive, but there is nothing "possessive" about this sentence.