A prepositional adverb takes the form of a preposition, which means it is typically the same type of word, but it functions in a sentence as an adverb. For example, in the sentence "He fell down," the word "down" is much like a preposition as used in a sentence like "Walk down the stairs." In the previous example, however, it is functioning as an adverb describing the verb "fell," rather than providing information about location. A prepositional adverb is often found at the end of a sentence and is not followed by an object that is part of a prepositional phrase with it, such as "stairs" in the second example.

The basic elements of any type of word are its form and function, which describe the appearance of a word and how it acts in a sentence. A prepositional adverb has the form of a preposition, can cause some confusion for new language speakers. This means they are often words like "in," "on," and "around," which are typically used as prepositions in a sentence. Unlike other uses, however, a prepositional adverb does not include an object after it providing details about where something is positioned or happening, such as the prepositional phrases "on the table" or "under the bed."

A prepositional adverb functions like an adverb within a sentence, which means that it modifies or describes a verb and the action taking place. For example, in the sentence "We should walk outside," the word "outside" is a prepositional adverb that modifies the verb "walk." This is the same function as an adverb like "quickly" in the sentence "We should walk quickly," except it takes the form of a preposition rather than the familiar adverb form ending in "-ly". In both sentences, however, the adverb provides additional information and modifies the verb, and in these two examples they both come after the verb.

One way in which a prepositional adverb is different from other types, however, is that adverbs can usually be moved around within a sentence. The example above could be rewritten as "We should quickly walk," and still make sense. A prepositional adverb, however, cannot be moved around since it has the form of a preposition; "We should outside walk," may still be understandable but is grammatically incorrect and awkward to read or say. Prepositional adverbs typically come at the end of a sentence and are not followed by an object, as standard prepositions are. "Think outside the box," uses "outside" as a preposition, requiring the object "box" for meaning.