In grammar, inflection or inflexion is the modification of a word to express different grammatical categories such as tense, grammatical mood, grammatical voice, aspect, person, number, gender and case. Conjugation is the inflection of verbs; declension is the inflection of nouns, adjectives and pronouns.
Inflection can be overt or covert within the same language. An overt inflection expresses grammatical category with an explicitly stated suffix. The Latin ducam, meaning "I will lead", includes an explicit suffix, -am, expressing person (first), number (singular), and tense (future). This is an overt inflection. In English, the word "lead" is not marked for either person or number, and is only marked for tense in opposition to "led" (i.e. is not specifically future tense). The whole clause, however, achieves all the grammatical categories by the inclusion of extra words. This is covert inflection (or periphrasis).
The process typically distinguishes lexical items (such as lexemes) from functional ones (such as affixes, clitics, particles and morphemes in general) and has functional items acting as markers on lexical ones.
Lexical items that do not respond to overt inflection are invariant or uninflected; for example, "must" is an invariant item: it never takes a suffix or changes form to signify a different grammatical category. Its category can only be determined by its context. Uninflected words do not need to be lemmatized in linguistic descriptions or in language computing. On the other hand, inflectional paradigms, or lists of inflected forms of typical words (such as sing, sang, sung, sings, singing, singer, singers, song, songs, songstress, songstresses in English) need to be analyzed according to criteria for uncovering the underlying lexical stem (here s*ng-); that is, the accompanying functional items (-i-, -a-, -u-, -s, -ing, -er, -o-, -stress, -es) and the functional categories of which they are markers need to be distinguished to adequately describe the language.
Constraining the cross-referencing of inflection in a sentence is known as concord or agreement. For example, in "the choir sings", "choir" and "sings" are constrained to the singular number; if one is singular, they both must be.
Languages that have some degree of overt inflection are inflected languages. The latter can be highly inflected, such as Latin (overtly), or weakly inflected, such as English (covertly), depending on the presence or absence of overt inflection. And, historically, English was traditionally described as a non-inflected Indo-European language.
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