How did the split between ichidan and godan verb conjugations come to be? Were suru and kuru always irregular?


Ichidan verbs ditch ru, godan verbs replace the next character with any of the
5 vowels. Is it known how this distinction came to be? Were the irregular
verbs kuru and suru always irregular?




The theory is this:

Godan verbs have a consonant at the end of their root. The various stem forms
each add a vowel on the end, and then further suffixes are added. This has
been the case in Japonic as long as we can tell; though /o/ is a new vowel to
the paradigm (they used to be called yodan verbs).

Ichidan verbs with -iru (kami-ichidan verbs) probably had a vowel at the end
of their root, initially. I'm not sure why the shuushikei added a -ru; this
might have been on analogy with the rentaikei (which uses -ru in several other
verb classes, and later replaced the shuushikei forms in most cases). I don't
think anyone's ever explored the question of why the root-final vowel is
always -i, though.

It's not fully clear whether or not all the vowels godan verbs take could have
at one point been added to ichidan forms. Old Japanese is very aggressive
about deleting vowels adjacent to other vowels, though, and so it wouldn't be
surprising if they had been there and were just all deleted. (I don't think
Ryuukyuuan languages show any trace of these vowels either, though.)

Ichidan verbs with -eru (shimo-ichidan verbs) were all once nidan verbs, which
had two vowels in the stem: -e- in most forms, but a shuushikei in -u and a
rentaikei in -uru (so modern 食べる was once 食ぶ in the shuushikei, and 食ぶる in the
rentaikei). These are mostly believed to come from the incorporation of 得る
'get', which before the creation of nidan verbs would have just been
irregular. These have since lost the alternations involving an -u, and have
become shimo-ichidan verbs.

(The only historical shimo-ichidan verb was 蹴る, which is now a godan verb - go
figure.)

する and くる seem to have been irregular as long as we can tell. People have
tried to propose various unified underlying root forms, but none have been all
that plausible. They're a bit less regular in Modern Japanese than they used
to be, but even so, the vowel alternations they underwent in Old Japanese
aren't the same as the vowel alternations for yodan verbs.

Incidentally, する and くる aren't the only irregular verbs in Modern Japanese. 行く
has the unexpected forms 行った and 行って instead of *行いた and *行いて, and 良い has an
irregular shuushikei/rentaikei いい instead of the expected (and historical) よい.

