You may have forgotten much of the grammar you were taught in school, if you were taught any at all, but most people can remember the parts of speech, at least the major ones. What is a noun? You probably said "a noun is a person, place, or thing." A verb? It describes an action, right? What about a preposition? You may have had more difficulty here, but perhaps you learned that prepositions tell you what an airplane can do to a cloud (go through, under, into, etc.). All of these definitions are well-entrenched in our educational system, but linguists are happy with none of them. If we scrutinize them, the traditional parts of speech turn out to be problematic. Consider the traditional definitions of noun and verb:
Noun: A noun is a person, place or thing. Verb: A verb describes an action or state of being.These definitions cover what we might call prototypical cases. Nouns often do label objects in the real world (car, tree, apple, etc.) and verbs most commonly express action (run, play, eat, etc.). But what do we do with abstract nouns like love or destruction. One easy way out is to add "idea" to the definition, but this change comes at a severe cost, for "idea" can be taken to encompass just about everything. Consider sentences such as
(1) John gave him a shove. (2) John shoved him.What allows us to say that shove in sentence (1) functions as a noun, but shoved in sentence (2) functions as a verb? The meaning of both sentences, after all, is essentially the same. And how do we account for verbs like hear or undergo? In a sentence like
(3) Vivica underwent a tonsillectomy as a child.the subject does not really perform an action, nor does the verb describe a mere state of being. It actually describes a change of state. If we broaden our definition to say that a verb tells us something about some person or thing, it becomes difficult to explain the difference between verbs and adjectives. The traditional definitions of parts of speech founder because they look for semantic definitions. These definitions may cover the canonical situations acceptably, but any definition that covers all cases becomes so vague as to be useless for making discriminations. Another problem with the common way of presenting parts of speech stems from their origins in Latin grammar. The term part of speech, and most of the labels themselves, were borrowed from the study of Latin. When English was first subjected to grammatical analysis, Latin was the language of educated Europeans, and it was presumed to represent an ideal, logical grammar. Therefore the earliest writers of English grammar books simply applied the terminology and classification they knew from Latin to the description of English. Because the two languages have significant grammatical differences, however, the fit was not perfect. Most Latin grammars described eight parts of speech: nouns, pronouns, verbs, adjectives, adverbs, prepositions, conjunctions, and interjections. If one didn't look too closely at the details, these categories worked, more or less, for English. But there were many problematic cases that troubled grammarians from the start. How, for example, should one handle the word the, or the word to when it appears in front of a verb? Latin had no direct equivalent to either word, but some grammarians tried to force these words to fit the Latin categories anyway. Therefore the was considered an adjective and to was called a preposition. Other grammarians disagreed, creating new categories for these words. This disagreement was never resolved in traditional grammar, and to this day, different textbooks make conflicting statements about these words. --- Notes  "Part of speech" is a literal translation of the Latin pars orationis.  Today, the reason some textbooks differ is likely that they have been influenced by more recent linguistic grammars, but even in the nineteenth century there was never perfect consensus. See, for example, Goold Brown, The Grammar of English Grammars, 6th ed. (1862), who argues for ten parts of speech. This lack of consensus is worthy of note because some textbooks confidently speak of eight parts of speech as if the whole issue had been settled centuries before.