General remarks on declension
Don't fear declensions in Old Norse. Their bark is louder
than their bite. There's a pattern. Always see the pattern.
I point out some of the symmetry explicitly but you should
try to find more yourself. It will ease the work on your memory
greatly and increase your understanding of the language as well.
The dative plural of all nouns ends with um.
The genitive plural of all nouns ends with a. Some of them
of them end with na.
There's a thick connection between the strong declension of nouns and the
definite declension of adjectives. Always see the pattern.