Parts of speech are the word categories of languages. Words belong to the same category if they show the same formal features or if they share a common function or position in a sentence. Both table and man are nouns because they show the possessive form (table’s, man’s) and by inflection. Both table and man can also fill a position in a sentence like “The ______ is big.” Nouns, verbs, adjectives, and adverbs can be defined by formal features, function, and position. Other parts of speech, such as prepositions and conjunctions, have no formal features. They can be defined by their function and position in a sentence.
Parts of speech were originally based on individual words. Often a multiword phrase or clause served the same function as a part of speech. For example, a prepositional phrase may act like an adverb. Compare “Mary slept late yesterday” and “Mary slept until noon yesterday.” The parts of speech are not rigid categories. Scholars disagree as to whether the first word in noun-noun compounds such as television set is a noun or an adjective. Also, new words are often constructed by shifting categories. For example, shortly after microwave ovens became available, the verb to microwave appeared in sentences such as John microwaved the frozen rolls.
Scholars differ on how to describe parts of speech. The traditional description lists eight classes: nouns, pronouns, verbs, adjectives, adverbs, prepositions, conjunctions, and interjections. Some scholars prefer to distinguish form classes–nouns, verbs, adjectives, and adverbs–from function words–prepositions, determiners, auxiliaries, and conjunctions. Others distinguish inflected classes from all other words, called particles. In addition, parts of speech were originally based on classical European languages and may not be adequate for describing all of the world’s languages.
