verbs - Why isn't ある's negative form あらない?

ある is listed in dictionaries as having ラ行五段活用, which would suggest a negative form of あらない. However, that form does not exist. Why not?

Answer

In modern Japanese, instead of the conjugation [未然形]{みぜんけい}+[無]{な}い, another word is used to express the plain negative, namely 無い.

This a process called suppletion, supplying a certain conjugational form with a different word. It exists in English as well. You don't say good and gooder, you talk about better, which comes from Proto-Indo-European *bhAd- good. Further examples include bad - worse, and be - is - was.

However, this kind of conjugation exists in Classical Japanese. Here the negative is expressed via 未然形+ず, and ある is no exception: あらず. ず , or perhaps better known in its form ぬ, is the helper verb of negation.

「[舟]{ふね}は[水]{みず}に[非]{あら}ざれば[行]{ゆ}かず。[水]{みず}、[舟]{ふね}に[入]{い}れば[則]{すなわ}ち[没]{ぼっ}す」

Even in modern Japanese, you can still say あらへん (Kansai dialect).

Lastly, although rare, there are attested instances of the form あらない (see 日本国語大辞典, entry あらない)

せく事はあらない

土佐【とさ】とて鬼の国でも蛇【へび】の国でもあらないものを

There is also あらなくに, but the な here is analyzed as the く-nominalization of ず.