I’m a little baffled by this correspondence, as mentioned on [this Imabi article](https://www.imabi.net/barecoveredforms.htm). It mentions an ancient *i* particle changing /a/ into /e/, but gives no reason for why there is a similar change from の to な in some compounds, or why 白（しろ）、黒（くろ）become しら、くら (cf. 暗闇・く**ら**やみ) in some contexts. I figured it might be を, but that feels rather tenuous.
Why is this?
3 comments
1. This is way beyond the level of /r/Japanese
2. The answer is not known. The i theory doesn’t work for adjectives, as you’ve noticed. One theory I’ve seen assumes a system of complex/compound vowels that merged in form A, and had the second vowel dropped in form B.
E.g.
1.sirau > siro (dipthong au merges to o)
2. sirau > sira (second part of compound vowel dropped)
Another theory is that the 被覆形 is simply older and remained only in certain contexts after a vowel shift produced the 露出形.
The reality is we will never know as this predates the oldest written records of Japanese.
This is a phenomenon that harkens back to Proto-Japonic, the ancestor of Japanese and Ryukyuan.
Basically, at least for the u—i, o—i and a—e correspondents, they were originally 2 vowels in a sequence, reconstructed as *ui & oi, *əi and *ai. When they are compounded, they lose the final *i*, so *mai “eye” —> *ma-pia “eye + LOC -> front”.
Then, during Pre-Old Japanese (before the 8th century), these two vowels merged into one. ui, oi, əi > i2; ai > e2, hence leading to **me2** vs **ma**pe1.
This article seems to attribute this phenomenon to the particle い which I am somewhat sceptical of since it is only really found in Western Old Japanese sections of Man’yōshu and some Imperial edicts and not found in any other Japonic languages, including the contemporary Eastern Old Japanese as well as being absent from both the Nihon Shoki and Kojiki, implying we should not trace it back to Proto-Japonic.
Unfortunately, it seems that the *o1* in しろ comes from [Proto-Japonic *o](https://en.m.wiktionary.org/wiki/Reconstruction:Proto-Japonic/siro) so I’m stumped on this, maybe u/matt_aegrin would know more
This Imabi article, as well as [the Wiktionary article on い](https://en.wiktionary.org/wiki/%E3%81%84#Etymology_3), is based on very-outdated scholarship about the particle い, with claims that are now considered outlandish.
**The facts on the ground about particle い are:**
* It was used in Western Old Japanese and early-Heian Middle Japanese, roughly 700-1000 AD.
* It was usually written as 伊.
* It marks the subject of a verb, and apparently only active-participant subjects.
* The modern Ōita dialect of Japanese has nominative case marker い, which marks all subjects. It may or may not be related to the much earlier い.
* No language or dialect within Japonic–aside from Western OJ and Ōita Modern Jp–attests such a particle.
* There is a [Middle Korean nominative case marker 이](https://en.wiktionary.org/wiki/-%EC%9D%B4) *i*, which developed from an older ergative case marker.
In contrast, **there is no so-called “emphatic particle い.”** The three examples [given on Wiktionary as definition #2](https://en.wiktionary.org/wiki/%E3%81%84#Etymology_3) are erroneous:
* The first involves 不絶射 *tayezi i* (絶えじ い), where *i* is either the above case marker い or a noun meaning “person” ([borrowed from Korean](https://en.wiktionary.org/wiki/%EC%9D%B4#Etymology_1_2)).
* The latter two mis-identify 伊間 *ima* (phonetic spelling of 今) as *-i ma* (い間).
As for words that **alternate between a pure vowel in a “bound form” and a diphthong in a “free form”,** the situation is exactly as u/Henrywongtsh described. To elaborate, there are five types of alternations for such words, which are all reconstructed as different Proto-Japonic diphthongs:
|Proto-Japonic|Western Old Japanese|Proto-Ryukyuan|WOJ Example|
|:-|:-|:-|:-|
|*a ~ *ai|a ~ e₂|*a ~ *e|天 ama ~ ame₂|
|*u ~ *ui|u ~ i₂|*u ~ *i|身 mu ~ mi₂|
|*o ~ *oi|u ~ i₂ (*rarely* o₁ ~ i₂)|*o ~ *i|黄 ku ~ ki₂ (*but compare Middle Japanese* 黄金 **ko**gane)|
|*ə ~ *əi|o₂ ~ i₂ (*rarely* o₂ ~ e₂)|*o ~ *e|木 ko₂ ~ ki₂|
|*a ~ *au|a ~ o₁|*a ~ *o|白 sira ~ siro₁|
The “bound forms” can occur only:
* As the initial components in compounds, or
* before genitive/possessive markers like *no₂* and *tu* (which can be thought of another kind of compounding)
…But “free forms” can also appear in these places as well. As a result, as far as frequency goes, the “free forms” reign supreme in all Japonic languages without exception, and all modern languages have done away with “bound forms” almost entirely… outside of a select few lingering prefixes like *sira-* “white” in Japanese (vs. bound *siro*), or 与那 *yuna-* “grainy, sandy” in Okinawan (vs. bound *yuni*; cognate to Japanese 米 *yone* “rice grain”).
For words that are **not** nominalized verbs, these alternating **i* and **u* of **Vi* and **au* are now usually believed to be genuine/original parts of the words, and that the final vowel somehow got dropped in certain types of compounds. (For verbs, there is a Proto-Japonic suffix *-ai- ~ *-i- which, when nominalized, can result in *i getting added to a word that didn’t originally have it–hence why I’ve excluded verbs here. An example like this would be 凪 *nagi₂* < *naNko-i-, formed from the same root as 和やか *nagoyaka* < *naNko-ya-ka.)
​
**To summarize the argument distinguishing the agentive/nominative case marker い from Proto-Japonic *Vi diphthongs:**
* The case marker い is restricted to Old/EarlyMiddle Japanese and Ōita-ben, and it looks very much like a Korean ergative/nominative case marker.
* Western Japan received many thousands of refugees from Baekje (their BFF in Korea), especially after the latter fell to Silla in the 660s AD.
* Words exhibiting **V ~ *Vi* occur everywhere in Japonic, including Hachijo and Ryukyuan, which have little to no Korean influence at all.
* The alternation of **V ~ *Vi* in Japonic is unrelated to being agent/subject, and instead has to do with compounding.
* There is no case particle **う that could explain the alternation *a ~ *au.
**Thus, it’s pretty straightforward to conclude that the case marker い is unrelated to the *i that occurs in *V ~ *Vi alternation.**