Part of speech
In grammar, a part of speech (also a word class, a lexical class, or a lexical category) is a linguistic category of words (or more precisely lexical items), which is generally defined by the syntactic or morphological behaviour of the lexical item in question. Common linguistic categories include noun and verb, among others. There are open word classes, which constantly acquire new members, and closed word classes, which acquire new members infrequently if at all.
Almost all languages have the lexical categories noun and verb, but beyond these there are significant variations in different languages. For example, Japanese has as many as three classes of adjectives where English has one; Chinese, Korean and Japanese have nominal classifiers whereas European languages do not; many languages do not have a distinction between adjectives and adverbs, adjectives and verbs (see stative verbs) or adjectives and nouns, etc. This variation in the number of categories and their identifying properties entails that analysis be done for each individual language. Nevertheless the labels for each category are assigned on the basis of universal criteria.
The classification of words into lexical categories is found from the earliest moments in the history of linguistics. In the Nirukta, written in the 5th or 6th century BC, the Sanskrit grammarian Yāska defined four main categories of words:
- nāma – nouns or substantives
- ākhyāta – verbs
- upasarga – pre-verbs or prefixes
- nipāta – particles, invariant words (perhaps prepositions)
These four were grouped into two large classes: inflected (nouns and verbs) and uninflected (pre-verbs and particles).
- peyar (noun),
- vinai (verb),
- idai (part of speech which modifies the relationships between verbs and nouns) and
- uri (word that further qualifies a noun or verb)
A century or two after the work of Nirukta, the Greek scholar Plato wrote in the Cratylus dialog that "... sentences are, I conceive, a combination of verbs [rhēma] and nouns [ónoma]". Another class, "conjunctions" (covering conjunctions, pronouns, and the article), was later added by Aristotle.
By the end of the 2nd century BC, the classification scheme had been expanded into eight categories, seen in the Art of Grammar (Τέχνη Γραμματική) :
- Noun: a part of speech inflected for case, signifying a concrete or abstract entity
- Verb: a part of speech without case inflection, but inflected for tense, person and number, signifying an activity or process performed or undergone
- Participle: a part of speech sharing the features of the verb and the noun
- Interjection: a part of speech expressing emotion alone
- Pronoun: a part of speech substitutable for a noun and marked for a person
- Preposition: a part of speech placed before other words in composition and in syntax
- Adverb: a part of speech without inflection, in modification of or in addition to a verb, adjective, clause, sentence, or other adverb
- Conjunction: a part of speech binding together the discourse and filling gaps in its interpretation
Traditional English grammar is patterned after the European tradition above, and is still taught in schools and used in dictionaries. It names eight parts of speech: noun, verb, adjective, adverb, pronoun, preposition, conjunction, and interjection (sometimes called an exclamation).
Since the Greek grammar of 2nd century BC, parts of speech have been defined by morphological, syntactic and semantic criteria. However, there is currently no generally agreed-upon classification scheme that can apply to all languages, or even a set of criteria upon which such a scheme should be based.
Linguists recognize that the above list of eight word classes is drastically simplified and artificial. For example, "adverb" is to some extent a catch-all class that includes words with many different functions. Some have even argued that the most basic of category distinctions, that of nouns and verbs, is unfounded, or not applicable to certain languages.
English words have been traditionally classified into eight lexical categories, or parts of speech (and are still done so in most dictionaries):
- any abstract or concrete entity; a person (police officer, Michael), place (coastline, London), thing (necktie, television), idea (happiness), or quality (bravery)
- any substitute for a noun or noun phrase
- any qualifier of a noun
- any action (walk), occurrence (happen), or state of being (be)
- any qualifier of an adjective, verb, clause, sentence, or other adverb
- any establisher of relation and syntactic context
- any syntactic connector
- any emotional greeting (or "exclamation")
Although these are the traditional eight English parts of speech, modern linguists have been able to classify English words into even more specific categories and sub-categories based on function.
The four main parts of speech in English, namely nouns, verbs, adjectives and adverbs, are labelled form classes as well. This is because prototypical members of each class share the ability to change their form by accepting derivational or inflectional morphemes. The term form is used because it refers literally to the similarities in shape of the word in its pronunciation and spelling for each part of speech.
Neither written nor spoken English generally marks words as belonging to one part of speech or another, as they tend to be understood in the context of the sentence. Words like neigh, break, outlaw, laser, microwave, and telephone might all be either verb forms or nouns. Although -ly is a frequent adverb marker, not all adverbs end in -ly (-wise is another common adverb marker) and not all words ending in -ly are adverbs. For instance, tomorrow, fast, very can all be adverbs, while early, friendly, ugly are all adjectives (though early can also function as an adverb). Verbs can also be used as adjectives (e.g. "The astonished child watched the spectacle unfold" instead of the verb usage "The unfolding spectacle astonished the child"). In such cases, the verb is in its participle form.
In certain circumstances, even words with primarily grammatical functions can be used as verbs or nouns, as in "We must look to the how's and not just the why's" or "Miranda was to-ing and fro-ing and not paying attention".
Functional classification 
The study of linguistics has expanded the understanding of lexical categories in various languages and allowed for better classifying words by function. Common lexical categories in English by function may include:
- Closed word classes:
See also 
- Grammatical category
- Part-of-speech tagging
- Quirky subject
- Sliding window based part-of-speech tagging
- Syntactic category
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- Cratylus 431b
- Beauzée, Nicolas, Grammaire générale, ou exposition raisonnée des éléments nécessaires du langage. (Paris, 1767).
- Zwicky, Arnold (30 March 2006). "What part of speech is "the"". Language Log. Retrieved 26 December 2009. "...the school tradition about parts of speech is so desperately impoverished"
- Hopper, P; Thompson, S (1985). "The Iconicity of the Universal Categories 'Noun' and 'Verbs'". In John Haiman. Typological Studies in Language: Iconicity and Syntax 6. Amsterdam: John Benjamins Publishing Company. pp. 151–183.
- Broschart, Jürgen (1997). "Why Tongan does it differently: Categorial Distinctions in a Language without Nouns and Verbs". Linguistic Typology 1 (2): 123–165. doi:10.1515/lity.19126.96.36.199.
- Klammer, Thomas; Schulz, Muriel R.; Della Volpe, Angela (2009). Analyzing English Grammar (6th ed.). Longman.
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- The parts of speech
- Parts of Speech Activities at Quia
- Guide to Grammar and Writing
- Martin Haspelmath. 2001. "Word Classes and Parts of Speech." In: Baltes, Paul B. & Smelser, Neil J. (eds.) International Encyclopedia of the Social and Behavioral Sciences. Amsterdam: Pergamon, 16538-16545. (PDF)