To identify a prepositional phrase, look for a preposition that begins a sentence fragment and is dependent upon the other words in the group, such as "to the library" or "under the bridge." The phrase may contain nouns and pronouns but should not contain a subject or verb. Determine whether the prepositional phrases behaves as an adjective that modifies a noun or pronoun or an adverb that modifies an adjective, adverb or verb.
Look for common prepositions, such as "in," "at," "near," "around" and "before," and pay attention to any verbs that come directly ahead of them. Many prepositions act as "particles" that are part of a compound verb. In the sentence "He turned down the volume," the words "turned down" act as one verb. In the sentence "He turned down a side street," the phrase "down a side street" acts as one adjective modifying the verb "turned."
In the previous example, "the volume" is the direct object of the verb "turned down" because it receives the action. A prepositional phrase may have its own object, but it never contains the direct object of the verb. Consider this example: "Stacy rode her bike on the grass." The bike is receiving the action and is the direct object of the verb "rode." The grass is the object of the preposition "on," and the entire prepositional phrase is an adverb that answers the question of where Stacy rode her bike.