User Hakaku on my thread about the last post on reddit’s /r/linguistics subreddit pointed out that there is perhaps a better analysis than the one I presented last time, and I am inclined to agree.
One thing I left out last time was the fact that I only dealt with consonant-stem verbs. Japonic languages also have vowel-stem verbs, and any adequate explanation must work for both. My previous solution does not. So here I present a better solution which should account for all regular forms. Here we’ll just work with the following roots: *kak- ‘to write’ (a consonant-stem verb), *kir- ‘to cut’ (another consonant-stem verb), and *ki- ‘to wear’ (a vowel-stem verb).
Note that vowel-stem verbs historically only ended in *-i or *-e, but this second group disappeared in Okinawan due to the Northern Ryūkyūan chain shift (or perhaps paradigm leveling with the former type, or both—the number of verbs ending in *-e in Japonic is quite small).
We start off much the same as last time, with the optionality of the glide at the beginning of *wor-*. In all forms, this creates a non-identical vowel cluster, which is illicit.
(1a) *kak-i+wor-um |write-INF+CONT-RLS| > *kak-i+or-um
(1b) *kir-i+wor-um |cut-INF+CONT-RLS| > *kir-i+or-um
(1c) *ki+wor-um |wear\INF+CONT-RLS| > *ki+or-um
This vowel cluster is resolved with V2 elision, where the *o in *or- is elided:
(2a) *kak-i+or-um > *kak-i+r-um
(2b) *kir-i+or-um > *kir-i+r-um
(2c) *ki+or-um > *ki+r-um
Next, a special change affects just the verb ‘to cut’. Throughout Okinawan, sequences of *ri became *i. This again creates an illicit vowel cluster, which is resolved with elision.
(3b) *kir-i+r-um > *ki-i+r-um (*ri to *i) > *ki+r-um (V2 ellision)
Next is the progressive palatalization/lenition of *r to *y.
(4a) *kak-i+r-um > *kak-i+y-um
(4b) *ki+r-um > *ki+y-um
(4c) *ki+r-um > *ki+y-um
Next is the palatalization of *ki to *t͡ɕi:
(5a) *kak-i+y-um > *kat͡ɕ-i+y-um
(5b) *ki+y-um > *t͡ɕi+y-um
(5c) *ki+y-um > *t͡ɕi+y-um
Next is a change that only affects ‘to write’, the coalescence of the *iy sequence to just palatalization on the proceeding consonant:
(6a) *kat͡ɕ-i+y-um > *kat͡ɕ+um
This complete coalescence could not occur in (6b) and (c) as it would obliterate the original root, which only is licit in the most frequent (in other words, irregular) verbs.
Penultimately, though this change is late it doesn’t necessarily have to be this late, the neutralization of syllable-final *m. and *n.:
(7a) *kat͡ɕ+um > *kat͡ɕ+un
(7b) *t͡ɕi+y-um > *t͡ɕi+y-un
(7c) *t͡ɕi+y-um > *t͡ɕi+y-un
Lastly, there is the morphological reanalysis of the auxiliary as part of the root, and the realis suffix as also having an imperfective meaning (expanding to take on an expanded version of the continuous function of the original auxiliary):
(8a) *kat͡ɕ+un > kat͡ɕ-un
(8b) *t͡ɕi+y-un > t͡ɕiy-un
(8c) *t͡ɕi+y-un > t͡ɕiy-un
As I mention in my reply on reddit, the one thing I don’t have a good explanation for is for “Naha” (lower class/innovative) forms like t͡ɕi-in ‘to cut, to wear’, It likely does not have anything to do with pitch accent (in some cases, a low pitch accent causes a vowel to lengthen in Naha but not Shuri), but instead likely caused by the difference in intensity between *i and *u, along with compensatory lengthening and morphological reanalysis.
(9a) kat͡ɕ-un (Naha as in Shuri)
(9b) *t͡ɕiy-un > *t͡ɕi-un (coalesence of *i and *y) > *t͡ɕi-n (V2 elision) > *t͡ɕii-n (compensatory lengthening) > t͡ɕi-in
(9c) *t͡ɕiy-un > *t͡ɕi-un (coalesence of *i and *y) > *t͡ɕi-n (V2 elision) > *t͡ɕii-n (compensatory lengthening) > t͡ɕi-in
reddit user Hakaku proposes the widely re-ocurring change of *yu to *i, which is probably a more straightforward analysis. We’d end up with:
(10) *t͡ɕiy-un > *t͡ɕi-in (coalesence of *yu to *i) > t͡ɕi-in (reanalysis, Naha form)