> > e-m = einhverr
> > e-n = einhvern
> > e-s = einhvers
> > e-t = eitthvert
> > e-u = eithverju

Hmm, there've been a few suggestions on how to decline this word. I hope no one'll mind if I make a few corrections here. It's a tricky one ;-)

e-m = einhverjum (masculine dative)
e-n = einhvern (masculine accusative)
e-s = einhvers (masculine/neuter genitive)
e-t = eitthvert (neuter nominative/accusative)
e-u = einhverju (neuter dative)

(All singular.)

In the nominative, unusually for Old Norse compunds, the first element is declined as well as the second: <einnhverr> (masculine), <einhver> (feminine), <eitthvert> (neuter). In other cases, <ein-> is used, and the last element is declined just like <hverr> "who, which" when it stands alone.