Given that the plural of Hungarian hó 'snow' is havak, one can guess that the plurals of szó 'word' and tó 'lake' are szavak and tavak.

But the plural of ló 'horse' is ... lovak, not *lavak.

And the plural of bíró 'judge' is either bírók (like another plural szók of szó 'word'*!) or bírák, not *bíravak or *bírovak.

As far as I know, there are six categories of nouns in Hungarian. Irregularities are in bold.

Noun class Nominative singular Nominative plural Accusative singular 3rd sg. possessive
ó (regular) hajó hajók hajót hajója
ó ~ a ajtó ajtók ajtót ajtaja
ó ~ á (only bíró) bíró bírók ~ bírák bírót ~ bírát (rare) bírója ~ bírája
ó ~ av (only szó) szó szók ~ szavak szót szava
ó ~ av (only and tó) havak havat tava
ó ~ ov (only ló) lovak lovat lova

What is the origin of these irregularities?

Can the last three classes be reconstructed with *-av and *-ov that merged into in final position? szók and szót might have originated as regularizations.

Can the ó alternating with á and a have been a long *ɔː corresponding to short a [ɔ] which no longer has a long counterpart?

*According to Edith Moravcsik (2003: 245),

As in the case of German Worte and Wörter, the first [plural szavak] refers to connected words in a text and the second [plural szók] refers to isolated words as in a dictionary.

Hence a German dictionary is a Wörterbuch 'words-book' (rather than a *Wortebuch), though a Hungarian dictionary is a szótár 'word-storage place'.

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