Every propositional phrase ends with an object of the preposition. The object of the preposition is always a noun. Look at these prepositional phrases:
down the stairs
in the corner
over the bridge
under the table
by the door
The last word of each phrase is a noun. That noun is called the object of the preposition.
Here are sentences using the prepositional phrases:
The ball rolled down the stairs.
My dog is sitting in the corner.
Dad drove over the bridge.
My napkin fell under the table.
Set the groceries by the door.
The object of a preposition is always a noun.
▶It’s your turn. Find the noun in this sentence that is being used as the object of the preposition. I put your book on the desk. Did you figure it out? Good for you!