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.
Nouns
In the protolanguage, nouns of different classes were characterized primarily by different suffixes and sometimes also by different endings. In the course of development of the Germanic languages, the various suffixes often merged with endings by means of various phonological processes, and eventually disappeared as independent morphemes. The new endings, which in early Germanic were still quite different from each other, in the course of development were affected by special Germanic phonological rules (Verner’s law, reduction of unstressed vowels) and in several instances became homorganic, cf. feminine ō-stem genitive ending *-ōR (-- *-ās) and u-stem genitive ending *-ōR (-- *-ous), or OIc. masc. ija-stem ending -ir (-- *-ijaz) and r-stem ending -ir (-- *-ēr). Many endings can also be added to nouns of more than one gender, cf. the very common endings -i and -r: han-i ‘rooster’ (masc.) vs. lyg-i ‘lie’ (fem.) vs. kvæð-i ‘poem’ (neutr.), or hest-r ‘horse’ (masc.) vs. brúð-r ‘bride’ (fem.), etc. In numerous cases, the ambiguity of endings caused confusion of inflections, transfer of nouns from one class to another, or paradigmatic split.
Some of the old endings were lost altogether, and were replaced by endings which originally belonged to other stems. Thus, e.g., the original genitive ending of i-stems was replaced to some extent by the ending of u- resp. ō-stems, and to some extent by the ending of a-stems. As a result, due to similarity of newly arisen i-stem genitives to the genitives of other nominal classes, entire new paradigms were built to many original i-stems. However, in the course of the processes of paradigmatic levelling, analogy and contamination, other stem classes occassionally adopted certain endings which originally belonged to i-stems.