Prepositional Phrases
Prepositions are words that help show the relationship of one noun to another or help show the relationship of two parts of a sentence. The sentence "I am sitting at my desk" has a preposition word "at" the shows the relationship of the action of "I" to a particular location, the "desk." The phrase, then, "at my desk" is a prepositional phrase, which will always extend from the preposition itself to the object of that preposition. In this case, the prepositional phrase answers the question of 'where' am I sitting. Since it modifies the verb by explaining where the action takes place, the entire phrase works in exactly the same way as an adverb. One that would work in exactly the same way as an adjective would be one that modified a noun or pronoun, as in the sentence "The collar of the dog needed to be replaced," where the prepositional phrase "of the dog" modifies "collar," a noun.