Heill, Haraldr!

Good questions.

> -Is it possible that numbers appear in dative or genetive case?

Sure! I just gave the first two cases for starters. All four
cases of the first four numbers are quite irregular but for 2-4
the dative and genitive are the same for all genders. The genders
are given in the order masculine-feminine-neuter.

nom. einn ein eitt
acc. einn eina eitt
dat. einum einni einu
gen. eins einnar eins

nom. tveir tvær tvau
acc. tvá tvær tvau
dat. tveimr
gen. tveggja

nom. þrír þrjár þrjú
acc. þrjá þrjár þrjú
dat. þrimr
gen. þriggja

nom. fjórir fjórar fjögur
acc. fjóra fjórar fjögur
dat. fjórum
gen. fjögurra

Øxn báru átta enni-tungl ok fjögur haufuð.
The oxen carried eight forehead-moons and four heads

> -In the sentence : "Menn Eiríks vega einn manna Óláfs" why the
> first "menn" is not in genetive case?

This corresponds to English usage; there are two genitives:

"one of Óláfr's men"

The name has the genitive suffix 's and we also have the genitive
preposition 'of'. This is perhaps even more clear in French:

"un des hommes d'Óláfr"

un = einn
des hommes = manna
d'Óláfr = Óláfs

> -I suppose that "bæði" is the nom. sing. of "báðir" (both), but if
> that is correct, why is not "bæðr"?

I seemed to remember having explained this word before - but maybe
that was in Færeyinga saga rather than the lessons. In any case 'bæði'
is not really a form of 'báðir' but a separate adverb derived from it.

Kveðja,
Haukur