In the sentence, "The river flows past the village," the word "past" is a preposition. It indicates the location or movement of the river in relation to the village, functioning to show that the river flows by or beyond the village. 
Here's why:
Prepositions show relationships:
Prepositions connect nouns or pronouns to other words in a sentence, often indicating a relationship of time, place, or direction. 
"Past" as a preposition:
In this case, "past" shows the river's movement in relation to the village. It forms a prepositional phrase "past the village," where "the village" is the object of the preposition. 
Examples:
The car drove past the house, and the river flows past the village are examples of "past" used as a preposition to show movement relative to an object.