Why is する considered irregular?


Yes, this sounds like a really confusing question. But I suddenly realized
that する seemed to be a perfectly regular 上二段活用 (kami nidan; upper bigrade)
verb with a stem of `s-`:

[code]

    未然形: し (as in しない)
    連用形: し (as in して, します)
    終止形: す (fossil in words like 愛す, 決す)
    連体形: する
    已然形: すれ (as in すれば)
    命令形: しろ (せよ seems mostly dead)
    
[/code]

Is this a coincidence, or does サ変 have any sort of etymological connection
with 上二段?




To add to the previous answer, there is no clear-cut distinction between
'regular' and 'irregular'; also, irregularities can often be explained and may
hint at an old form or conjugational system. Eg strong verbs in English and
German (ablaut conjugation), be-was-is-am (merger of different verbs).

As for ある, while *あらない is not used in modern Japanese, grammar (conjugational
patterns) would predict this form and I would not be surprised to find it in
some dialect. Also, we can negate ある via the 未然形: 非【あ】らず.

This slight irregularity in ある is considered insignificant enough to be
classified as a 'regular' verb, but never mind by which name we choose to call
it. There are also other slight irregularities: いらっしゃイます (not いらっしゃります), 乞ウた
(not こった), 行ッた(not いいた).

The origin of these different conjugation paradigms is unclear. According to
one hypothesis, many 一段 and 二段 verbs were formed by processes such as adding
ある, 得る; and that there is an older conjugation paradigm explaining irregular
forms.

Without going into details, observe that 3/4 of all verbs in Old Japanese are
四段; and a few 'irregular' words such 死ぬ・死ぬる・死に・死な(in the order 終・体・連・未 base)
hint at an older 四段-like conjugation with a 連体形 ending in る.

See 'Bjarke Frellesvig and John Whitman, 2008, Proto-Japanese: Issues and
Prospects' for a technical discussion (cited in his book 'A history of the
Japanese language', I have not read it.)

See <https://archive.org/stream/historicalgramma00sansuoft#page/150/mode/2up>
for a dated and less-technical discussion.

To summarize, it seems likely that your observations are not coincidence and
that there is a relation between the different conjugation paradigms, but the
details may be lost in the mist of time.




To add to the previous answer, there is no clear-cut distinction between
'regular' and 'irregular'; also, irregularities can often be explained and may
hint at an old form or conjugational system. Eg strong verbs in English and
German (ablaut conjugation), be-was-is-am (merger of different verbs).

As for ある, while *あらない is not used in modern Japanese, grammar (conjugational
patterns) would predict this form and I would not be surprised to find it in
some dialect. Also, we can negate ある via the 未然形: 非【あ】らず.

This slight irregularity in ある is considered insignificant enough to be
classified as a 'regular' verb, but never mind by which name we choose to call
it. There are also other slight irregularities: いらっしゃイます (not いらっしゃります), 乞ウた
(not こった), 行ッた(not いいた).

The origin of these different conjugation paradigms is unclear. According to
one hypothesis, many 一段 and 二段 verbs were formed by processes such as adding
ある, 得る; and that there is an older conjugation paradigm explaining irregular
forms.

Without going into details, observe that 3/4 of all verbs in Old Japanese are
四段; and a few 'irregular' words such 死ぬ・死ぬる・死に・死な(in the order 終・体・連・未 base)
hint at an older 四段-like conjugation with a 連体形 ending in る.

See 'Bjarke Frellesvig and John Whitman, 2008, Proto-Japanese: Issues and
Prospects' for a technical discussion (cited in his book 'A history of the
Japanese language', I have not read it.)

See <https://archive.org/stream/historicalgramma00sansuoft#page/150/mode/2up>
for a dated and less-technical discussion.

To summarize, it seems likely that your observations are not coincidence and
that there is a relation between the different conjugation paradigms, but the
details may be lost in the mist of time.

