Kartvelian languages such Georgian have a very complex agglutinative verb structure.
Georgian is very well studied but there's not a lot of self-study books or online sites that go really in depth. I do have several self-study books but they don't seem to cover everything, or some things are particularly hard to find.
I came across a verb "ატარებს" (atarebs) / "ტარება" (tareba), "to carry" which has an initial "a" in one of the citation forms (third person singular present indicative) but not in the other citation form (verbal noun).
But the form I encountered, "ეტარებინა" (etarebina) has an initial "ე".
There are several kinds of prefixes that I am aware of but I can't figure out what this is. It's not a preverb since that is a closed class. Georgian has a complex relationship between a concept called "version" and another of "polypersonal verbs". I understand that these are single vowel prefixes and I've seen "ე" mentioned but not thoroughly treated.
But Georgian is also famous for having many irregular verbs and lots of suppletion, and there could well be some other categories of the verb that I haven't even become aware of yet.
In my quest I discovered on Wiktionary that "ეტარებინა" is the third person singular pluperfect, but this doesn't help me to understand at all the process that led to the initial vowel changing!
Can anybody explain what is happening in this verb form?