A morpheme is the smallest meaningful ultimate unit which can’t be further analyzed into immediate constituents. Morphologically words are monomorphic and polymorphic (table; un ] [gent ] [ le ] [ man ] [ ly). A morpheme is an abstract unit comparable with a deep structure, which is unobservable. It realizes itself in allomorphs. The morpheme of plurality, for example, manifests itself in the following allomorphs: tables, crises, phenomena, children, knives, termini, formulae, sheer (a morphemic zero). When analyzing words morphologically, we roughly identify allomorphs with morphemes.
Morphemes are classified according to different criteria. According to meaning, morphemes are divided into lexical, grammatical and lexico-grammatical ones (table; table ] [ s; teach ] [ er). According to position, morphemes are divided into opening, and closing, internal and external. According to function, morphemes are divided into root, derivational (affixal) and inflexional. According to self-dependence, morphemes are divided into free (lexical), which build up words, bound (inflexional and derivational), which never occur in isolation, and semi-bound (word-morphemes), which look like words (be, have, shall, will, should, would), but function as inflexions.
The morphemic model of an English word is prefix – root – suffix – inflexion.