Participles and prepositions are the source of confusion for many who wish to better understand the mechanics of English grammar. Both elements of the language are used in phrases, and both function as modifiers. Notwithstanding, participles and prepositions -- for the most part -- play separate roles in the architecture of a sentence as they are separate parts of speech; prepositions are one part of speech, and participles are verb forms, otherwise known as verbals.

Participles

According to the Purdue Online Writing Lab, a participle is defined as a verb "that is used as an adjective and most often ends in -ing or -ed." Because you use participles as adjectives, they modify nouns and pronouns. Participles come in two forms: present participles and past participles. Present participles end with -ing and are often confused with gerunds. Past participles end with either -ed, -d, -t, -n, -en or -ne. For the sake of clarity, when using participles you should place them "as close to the nouns or pronouns they modify as possible."

Prepositions

The Writing Center at the University of Ottawa defines a preposition as a word that "links nouns, pronouns and phrases to other words in a sentence." They often signal spatial relationships between the words they objectify and the rest of the sentence. Common prepositions of this type are "in," "on," "at," "above" and "beneath." Prepositions can also indicate time-related affiliations, such as "during," "before," "after" and "until."

Participial Phrases and Prepositional Phrases

Both participles and prepositions function as parts of grammatical units called phrases. Participial phrases include the participle and the word it modifies, along with any noun phrases or pronouns "that function as the direct object(s), indirect object(s), or complement(s) of the action or state expressed in the participle," Purdue explains. Participial phrases, like participles by themselves, always function as adjectives. A prepositional phrase consists of the preposition, its object (or the word it presents) along with any correlative adjectives or adverbs. Prepositional phrases, unlike participial phrases, can serve as adjectives and adverbs.

Participial Prepositions

Aside from the fact that both parts of speech -- participles are verbals -- are used in phrases that function as modifiers, another point of confusion arises from the overlap, which comes in the form of the participial that plays the role of a preposition: "Provided the meeting starts on time, it should still be light out by the end." Here, "provided" is the participle. Grammarist.com ascribes the term "participial preposition" to this usage, and claims these participles have become accepted as prepositions because of "widespread, long-time usage habits," when they would otherwise be considered dangling modifiers. Common examples include "assuming," "barring," "considering," "during," "given," "notwithstanding," "provided," "regarding" and "respected."