The other day, my greek professor asked me on my opinion of the historial roots of the third person plural of εἰμί, εἰσί.
What at first occured to me was that is was formed by the stem *es and the personal ending -nti, i.e. *esnti, which then lost the s in the position before the n, ti becomes si, *ensi loses the n and becomes eisi because of compensatory lengthening. (I don't know about the order of the rules, so in no particular order)
when I got home, however, and checked my historical grammar, it said that the third person plural was formed on the stem *s-, that is *senti. This makes as much sense as my original explanation, except one thing: Where did the inital s go? Naturally, it would have become h-, but initial aspirations don't just disappear, do they?
My question, thus, is, how is the third person plural of εἰμί formed historically?