A prepositional phrase consists of a preposition followed by a noun phrase (its object), and functions as either an adjective or adverb within a sentence. Adjective prepositional phrases modify nouns (the book on the shelf); adverb prepositional phrases modify verbs or adjectives (She ran with great speed). They add detail about location, time, manner, and other relationships.
Bracket prepositional phrases in complex sentences, then determine whether each modifies a noun or verb. Practice moving adverb prepositional phrases to different positions to see how sentence rhythm changes.
You already know what prepositions are and what noun phrases are — a prepositional phrase (PP) combines them: a preposition followed by a noun phrase that serves as its object. The preposition expresses a relationship (location, direction, time, manner, purpose), and the noun phrase names the thing involved in that relationship. "On the shelf," "after dinner," "with remarkable speed" — each is a preposition plus its object, forming a unit.
That unit always plays one of two roles in a sentence. If it modifies a noun, it functions as an adjective and answers *which one?* or *what kind?*: "The book on the shelf" tells you which book. If it modifies a verb, adjective, or whole clause, it functions as an adverb and answers *how?*, *when?*, *where?*, or *why?*: "She ran with great speed" tells you how she ran. Identifying the function is simply a matter of asking what the PP is attached to.
Position matters for adjective PPs: they must stay close to the noun they modify or the sentence becomes ambiguous. "I saw the man with binoculars" is a famous example — did the man have binoculars, or did I? The PP "with binoculars" drifts away from its intended noun and accidentally latches onto the wrong one. This is why misplaced modifiers (a topic this one builds toward) are so often prepositional phrases.
The most consequential downstream consequence of PPs is their effect on subject-verb agreement. Because PPs frequently appear between a subject and its verb, they tempt writers to match the verb to the nearest noun rather than the actual subject: "The box of chocolates is here" — the subject is *box*, not *chocolates*, even though *chocolates* is closer to the verb. The prepositional phrase "of chocolates" is a modifier, not the subject. Learning to bracket and mentally remove PPs is the single most reliable technique for avoiding this agreement error.
A practical habit: when you encounter a noun followed by "of," "in," "on," "from," "with," or "by," immediately ask whether that noun is the subject or merely the object of a preposition. If it is the object of a preposition, it cannot be the subject. Developing this reflex will serve you in every sentence that involves description, qualification, or detail — which is nearly every sentence worth writing.