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]citation needed[, 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.
In linguistics, morphology is the identification, analysis and description of the structure of a given language's morphemes and other linguistic units, such as root words, affixes, parts of speech, intonation/stress, or implied context (words in a lexicon are the subject matter of lexicology). Morphological typology represents a method for classifying languages according to the ways by which morphemes are used in a language—from the analytic that use only isolated morphemes, through the agglutinative ("stuck-together") and fusional languages that use bound morphemes (affixes), up to the polysynthetic, which compress many separate morphemes into single words. (One of the definitions for Morphology).]clarification needed[
While words are generally accepted as being (with clitics) the smallest units of syntax, it is clear that in most languages, if not all, words can be related to other words by rules (grammars). For example, English speakers recognize that the words dog and dogs are closely related—differentiated only by the plurality morpheme "-s", which is only found bound to nouns, and is never separate. Speakers of English (a fusional language) recognize these relations from their tacit knowledge of the rules of word formation in English. They infer intuitively that dog is to dogs as cat is to cats; similarly, dog is to dog catcher as dish is to dishwasher, in one sense. The rules understood by the speaker reflect specific patterns, or regularities, in the way words are formed from smaller units and how those smaller units interact in speech. In this way, morphology is the branch of linguistics that studies patterns of word formation within and across languages, and attempts to formulate rules that model the knowledge of the speakers of those languages.
The verbal system of the Proto-Indo-European language (PIE) was a complex system, with verbs categorized according to their aspect — stative, imperfective, or perfective. The system utilized multiple grammatical moods and voices, with verbs being conjugated according to person, number and tense. The system of adding affixes to the base form of a verb (its root) allowed modifications so that it could form nouns, verbs, or adjectives. The verbal system is clearly represented in Ancient Greek and Vedic Sanskrit, which closely correspond in nearly all aspects of their verbal systems and are two of the most well-understood of the early daughter languages of Proto-Indo-European. Aside from the addition of affixes, vowels in the word could be modified in a process called ablaut. This is still visible in the Germanic languages (among others)—for example, the vowel in the English verb to sing varies according to the conjugation of the verb: sing, sang, and sung.
The system described here is known as the "Cowgill-Rix" system and, strictly speaking, applies only to what Don Ringe terms "Western Indo-European" (Western IE), i.e. IE excluding Tocharian and especially Anatolian. The system also describes Tocharian fairly well, but encounters significant difficulties when applied to Hittite and the other Anatolian languages. In particular, despite the fact that the Anatolian languages are the earliest-attested IE languages, much of the complexity of the Cowgill-Rix system is absent from them. In addition, contrary to the situation with other languages with relatively simple verbal systems, such as Germanic, there is little or no evidence of the "missing" forms having ever existed. Furthermore, many of the forms that do exist have a significantly different meaning from elsewhere. For example, the PIE perfect/stative conjugation shows up simply as a present-tense conjugation known as the ḫi-present, with no clear meaning; on the other hand, the PIE nu-present, which in other languages is a primary verb suffix with no clear meaning, is in Hittite a productive secondary verb suffix that forms causative verbs. (On the other hand, Germanic, among others, has a class of present-tense verbs derived from PIE perfect/stative verbs, and both Germanic and Balto-Slavic have a class of secondary n- verbs with a clear meaning, derived originally from nu- and/or neH- verbs, so it is possible that many of the Anatolian differences are innovations.) It is generally accepted that the Anatolian languages diverged from other IE languages at a point somewhat before the Cowgill-Rix system was fully formed; however, there is no consensus concerning what the inherited system looked like, and which Anatolian differences are innovations vs. archaisms.
Sesotho nouns signify concrete or abstract concepts in the language, but are distinct from the Sesotho pronouns.