Korean words can be divided into two classes: inflected and uninflected. The uninflected words are nouns, particles, adverbs and interjections. Inflected words are classed as action verbs, descriptive verbs, copula and existential. The distinction between action and descriptive verbs can be shown by the way in which paradigmatic forms such as propositive and processive are combined with verbal forms. For instance, a descriptive verb lacks propositive and processive forms. Thus, whereas the action verb plus the propositive ca or the processive nun is grammatical, the combinations of descriptive verbs with the same endings are not: mok-ca ‘let’s eat’ and mok-nun-da ‘is eating’ but *alumtap-ca ‘let’s be beautiful’ and *alumtap-nun-da ‘is being beautiful’. While the copula behaves like a descriptive verb, the existential behaves like an action verb with respect to conjugation; thus, *i-ca ‘let’s bet and *i-n-ta ‘is beeing’ are ungrammatical but is’-ca ‘let’s stay’ and is’-nun-ta ‘is staying’ are grammatical.
As predictable from the above discussion, each inflected form consists of a base plus an ending. Bases and endings can be classed into groups according to the ways in which alternant shapes of bases are combined with endings. There are two kinds of ending: one-shape endings such as -ko, -ta, -ci and -kes’ and two-shape endings such as -supnita/-upnita, -una/-na and -un/-n. Two-shape endings are phonologically conditioned alternants; thus, for instance, the formal form -supnita occurs only with base forms ending in a consonant, but the altemant form of the formal form /-upnita occurs only with base forms ending in a vowel. Based on these classes of endings, verb bases can be divided into two groups: consonant bases (i.e. bases ending in a consonant) and vowel bases (i.e. bases ending in a vowel). There are, however, some classes of bases whose final sounds are changed when attached to endings. Thus, in addition to regular bases which do not alter when combined with the ending, there are about five classes of consonant bases which alter with the ending: bases ending in t, bases ending in w, bases ending in h, bases ending in sonorants and s-dropping bases. Vowel bases have three classes in addition to the regular vowel bases: l-extending vowel bases, l-doubling bases and l-inserting vowel bases.