What's the difference between hvert and hver?
«Et år» is a neuter substantive, so whatever adjective you add to it, must be neuter too, in this case hvert instead of hver
Why is it ei? Is ordbook feminine?
'Bok' can be either feminine or masculine, as can 'ordbok' and any other compound noun which has 'bok' placed last.
Fascinating! Are there any other words that can be either masculine or feminine?