Does that mean that "Возле реки зоопарк" should be an accepted answer? Some questions want me to phrase it this way with the preposition first. Others want the subject first. I am having trouble understanding when to use which word order.
The order of the words. Your sentence would be translated as "The river is near the zoo" and not otherwise.
(Of course, it carries the idea, but the purpose is to deliver the closest translation possible, and "Возле реки зоопарк" is not one of them.)
I think that the Russian preposition “k” is always followed by the dative case, not the prepositional. But it’s easy to make that mistake, as the feminine noun that follows it has the same ending (-e) in both cases.