What is the difference in Russian pronunciation between вести and вѣсти?
I don't know what was the phonetic difference when 'ѣ' was originally put into use, but today I do not detect any difference in how words formerly spelled with 'ѣ' and 'е' sound. I do not know what the objections were in 1920's in that regard. I am certain, however, that at some point there was a phonetic distinction because one can see a trace of it in Bulgarian, and now you told me that in Serbioan the distinction is apparent.
"svyatost"
That, I assure you, is Я like any other. "Святий" is Church slavonic. The Russian is "святой", and Я is muted because it is not under stress. E or И would be likewise muted when unstressed, and all three would sound alike. This does not demonstrate anything about Я in particular. Consider "немой" (mute) and "витой" (twisted), the first syllable sounds the same in all three.
Neither is any long or short in Serbian, by itself. However, they become long and short in different words, and not for effect. The word нос (nose) or пост (fast, as in not eating) which have very long o, as opposed to стоj (stop), боj (battle), very short o.
I am certain, however, that at some point there was a phonetic distinction because one can see a trace of it in Bulgarian, and now you told me that in Serbian the distinction is apparent.
The disticntion is apparent in how it evolved. In the western dialects the "ѣ" evoled into "iye/ye" and in the more farther regions (in Dalmacia) into "и" (as in Unkarinian), while in Eastern Serbian, and Bulgarian it evoled into an "э", sometimes even into an "ээ"!
Thus, within the same language, words such as свѣт is pronounced as as свэт, свиjеt and свит, and spelled accordingly, rather than just свѣт.
My impresison is that the Bolshevik orthographic reform replaced the "ѣ" not only with "e" (that is ye, which is okay) but also with "я" in some words. I know that "ѣ" was a YE because the Serbian writer and vladyka from Montengro Petar Petrovich Nyegosh spelled his surname as Hѣгошъ (modern spelling Његош). So, how did it beocme a "я" in some words?
But on the other hand, a 12th century epistle to the people of Dubrovnik by a Bosnian lord was signed by the scribe Radoye as "Ѣ Радое" using the yat in place of Я!
and Я is muted because it is not under stress. E or И would be likewise muted when unstressed, and all three would sound alike
Yes, that's the case wiht modern Greek too. You have I, Y, and H all pronoiced as И. Of course nothing exemplifies this better than English.
I meant to get back to you on this. Свети is also Serbian. Serbian also has 7 cases just as Church Slavonic. Also the Serbian infinitive ending on -и is idnetical to Church Slavonic, as in молити (to pray). Although Church Slavonic sounds and looks "Russian," it is grammatically much closer to Serbian.