As I have written many times before, the line between grammatical forms is blurry at best, especially among lexical categories like adjectives and verbs. Adjectives are “words that describe nouns.” Verbs are “words that denote actions, occurrences, and states.” Participles are nonfinite verbs that share characteristics and functions of verbs, adjectives, and nouns. A present participle is a participle in the form of the -ing suffix affixed to a verb base. A past participle is a participle in the form of the -ed or -en suffix affixed to a verb base. Some grammars classify participles that function as noun phrase modifiers as participial adjectives.
The internal structure that distinguishes adjectives from other grammatical forms is degree of comparison. Prototypical adjectives express three degrees of modification: positive, comparative, or superlative. The positive form is the base form of the adjective. The comparative form expresses a comparison between two entities in quality, quantity, or degree. The superlative form expresses the highest degree of comparison.
The internal structures that distinguish verbs from other grammatical forms are number, person, tense, mood, and voice. Grammatical tense is a grammatical category that relates the time of an event to the time of an utterance. English has only two tenses: past and present. Prototypical English verbs express number and person in the third person singular of the simple present. Prototypical verbs have two participial forms: present participle and past participle.
Participles blur the line between verbs and adjectives. For example, the exciting book and the excited child are both noun phrases. The present participle exciting describes the noun book. The past participle excited describes the noun child. The words exciting and excited in the exciting book and the excited child are gradable. Constructions such as the more exciting book, the very exciting book, the most excited child, and the very excited child are possible. Both exciting and excited are therefore analyzable as adjectives.
The base form excite can also take subjects and objects. The exciting book means “the book excites.” The excited child means “something excites the child.” The noun phrase the book functions as the subject of the verb excites. The noun phrase the child functions as the direct object of the verb excites. Only verbs take subjects and objects. Verbs also have participial forms. Both exciting and excited are therefore also analyzable as verbs.
Other syntactic tests can tidy the line between adjective and verb between participial adjectives. Both adjectives and verbs can take complements. Only the adjective excited takes a complement in the form of a prepositional phrase. For example, in the child is very excited about the party and the child very excited about the party shrieked, the prepositional phrase about the party function as an adjective phrase complement to complete the meaning of the adjective excited. The noun phrase the child very excited about the party conveys the meaning “the party excites the child.” However, the verb excite does not take a complement in the form of a prepositional phrase. The word excited is therefore analyzable as an adjective.
Word order also distinguishes adjectives from verbs. In she read the exciting book and she read the book exciting the child, the word exciting modifies the noun book. The adjective exciting precedes the noun book. The verb exciting follows the noun book. The phrase exciting the child is a verb phrase. The verb exciting takes a direct object. The noun phrase the child functions as the direct object of exciting. Only verbs can take direct objects. The exciting in the exciting book cannot take an object. The book exciting the child is grammatically possible, but *the exciting the child book is not. The adjective exciting cannot take an object. Verbs that function as a noun phrase modifier typically follow the noun. Adjectives typically precede the noun.
As words that share characteristics and functions of verbs and adjectives, participial adjectives create a conundrum for grammatical analysis. Sometimes distinguishing the grammatical form of a participle is clearer. Only verbs take direct objects. Only adjectives exhibit gradability, specifically degrees of comparison. Other participles that function as noun phrase modifiers are more ambiguous in form. In either case, the participle modifies a noun.
Brinton, Laurel J. & Donna M. Brinton. 2010. The linguistic structure of Modern English, 2nd edn. Amsterdam: John Benjamins Publishing Company.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.