In grammar, inflection or inflexion – sometimes called accidence – is the modification of a word to express different grammatical categories such as tense, case, voice, aspect, person, number, gender, and mood. The inflection of verbs is also called conjugation, and one can refer to the inflection of nouns, adjectives and pronouns as declension. An inflection expresses one or more grammatical categories with a prefix, suffix or infix, or another internal modification such as a vowel change. For example, the Latin verb ducam, meaning "I will lead", includes the 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 one or more free morphemes (a unit of meaning which can stand by itself as a word), and one or more bound morphemes (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 subject to inflection are said to be invariant; for example, the English verb must is an invariant item: it never takes a suffix or changes form to signify a different grammatical category. Its categories can be determined only from its context. Requiring the forms or inflections of more than one word in a sentence to be compatible with each other 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, Greek, Spanish, Biblical Hebrew, and Sanskrit), 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.

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