KISS Grammar Workbooks
The Functions of Prepositional Phrases
     Most prepositional phrases function as adjectives, as adverbs, or both. Just as with regular adjectives and adverbs, to determine the function of a prepositional phrase you need to first find what it describes.
The squirrel {in the tree} was eating nuts.

The phrase "in the tree" describes the squirrel, and since "squirrel" is a noun, the prepositional phrase functions as an adjective. Compare this to:

The squirrel was eating nuts {in the tree}.

In this sentence, the phrase "in the tree" explains where the squirrel was eating. It therefore describes "was eating" and thus functions as an adverb.

     Sometimes a prepositional phrase can function as both an adjective and an adverb at the same time.

The squirrel wants nuts {for supper).

Some people will see "for supper" as explaining which nuts the squirrel wants. Thus they will explain the "for supper" as an adjective to "nuts." Other people will view the phrase as answering the question "Wants why? They will consider it to be an adverb to "wants." Either explanation is acceptable.

     Some prepositional phrases function as indirect objects:

The squirrel offered a nut to the owl.

In this sentence, "to the owl" functions just like the indirect object would in "The squirrel offered the owl a nut."