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.

An inflection expresses one or more grammatical categories with an explicitly stated prefix, suffix, or infix, or another internal modification such as a vowel change. For example, the Latin ducam, meaning "I will lead", includes an explicit suffix, -am, expressing person (first), number (singular), and tense (future). The use of this suffix is an inflection. In contrast, in the English clause "I will lead", the word "lead" is not inflected for any of person, number, or tense; it is simply the bare form of a verb.

The inflected form of a word often contains both a free morpheme (a unit of meaning which can stand by itself as a word), and a bound morpheme (a unit of meaning which cannot stand alone as a word). For example, the English word "cars" is a noun that is inflected for number, specifically to express the plural; the content morpheme "car" is unbound because it could stand alone as a word, while the suffix "s" is bound because it cannot stand alone as a word. These two morphemes together form the inflected word "cars".

Words that are never subjected to inflection are said to be invariant; 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.

Requiring the inflections of more than one word in a sentence to be compatible according to the rules of the language is known as concord or agreement. For example, in "the choir sings", "choir" is a singular noun, so "sing" is constrained in the present tense to use the third person singular suffix "s".

Languages that have some degree of inflection are synthetic languages. These can be highly inflected, such as Latin, or weakly inflected, such as English. Languages that are so inflected that a sentence can consist of a single highly inflected word (such as many American Indian languages) are called polysynthetic languages. Languages in which each inflection conveys only a single grammatical category, such as Finnish, are known as agglutinative languages, while languages in which a single inflection can convey multiple grammatical roles (such as both nominative case and plural, as in Latin and German) are called fusional. Languages such as Mandarin Chinese that never use inflections are called analytic or isolating.

Read more about Inflection:  Examples in English, Declension and Conjugation, Inflection Vs. Derivation, Inflectional Morphology

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