Morphological System
The morphological system of Old Icelandic is fairly complex. The nominal system consists of sixteen inflectional classes, most of which can be further subdivided into subclasses. The Old Icelandic adjectives can be grouped into two morphologically and semantically distinct classes of “strong” (indefinite) and “weak” (definite) adjectives. The verb system consists of three large classes of the so-called “strong”, “weak” and “preterito-present” verbs. Each of these classes can be subdivided into subclasses.
All of them were affected by various analogival changes, although in different word classes, analogy was reflected in different ways.
Verbs
The endings of Old Icelandic verbs, compared to the endings of nouns are relatively straightforward. In the Icelandic verb system, much more problematic are the morpho-logically and phonologically conditioned vowel alternations in the root, which can significantly affect the shape of the root in different forms of the same word. Analogical restorations and transformations, which always work against regular phonological development, have contributed to the creation of numerous by-forms and parallel paradigms.
Just like was the case with Old Icelandic nouns, also in the verb system there have been many transfers between verb classes and paradigmatic splits. Due to specific phonological shape, many archaic strong verbs developed in various irregular ways, eventually developing abnormal paradigms. Already at an early stage, native speakers created alternative regular paradigms for such verbs, and in many instances irregular verbs possess more than one paradigm (in certain cases as many as six, cf. the verb gørva ‘make’, which due to aberrant shape in the course od development acquired five by-forms, cf. gera, gerva, gøra, gj o˛ra, gj o˛rva, each having its own paradigm).
Also the morphologically conditioned vowel alternations (ablaut) underwent much re-shuffling. Of the original ablaut patterns, some were more productive than others, and in the course of development were adopted by those verbs which originally did not have them. Thus, the productive ablaut pattern of alternating full and zero grade of the root vowel, originally at home in the classes I-III of strong verbs, cf. skaut ‘shot’ (1./3. p. sg. pret.) vs. skutu (3. p. pl. pret.) occassionaly made its way into the VI and the VII classes, cf. óx ‘grew’ (1./3. p. sg. pret.; VI cl.) vs. uxu (3. p. pl. pret.) beside the original óxu. Likewise, beside the original jós – jósum ‘poured’ (VII cl.) there appeared jós – jusum. In one case, the original pair was also eliminated, cf. sveip – *sveipu ‘sweep’ (the latter form unattested) became sveip – svipu.