(1) a-plurals

SingularPlural
Nom.fisk-r (fish)fisk-ar
Acc.fiskfisk-a
Dat.fisk-ifisk-um
Gen.fisk-sfisk-a

42. So also heimr (home, world); konungr (king); Þōrr (Thor), acc. Þōr, gen. Þōrs; steinn (stone), acc. stein, gen. steins, pl. nom. steinar; hrafn (raven), acc. hrafn, pl. nom. hrafnar; þurs (giant), acc. gen. þurs, pl. nom. þursar.

43. Dissyllables in -r, -l, -n generally throw out the preceding vowel before a vowel-inflection: hamarr (hammer), dat. hamri; jǫtunn (giant), pl. nom. jǫtnar. kętill (kettle) and lykill (key) show unmutated vowels in the contracted forms, as in the acc. plur. katla, lukla.

44. Some nouns of this decl. take -ar in the gen. sing., especially proper names, such as Hākon, gen. Hākonar.

45. Some nouns add v before vowels: sær (sea), gen. sævar.

46. The dat. sometimes drops the i: (sea), Þōr. dagr (day) mutates its vowel in the dat. dęgi.

47. Nouns in -ir keep the i in the sing., and drop it in the plur.:

SingularPlural
Nom.hęlli-r (cave)hęll-ar
Acc.hęllihęll-a
Dat.hęllihęll-um
Gen.hęlli-shęll-a

48. So also a number of proper names, such as Skrȳmir, Þōrir.