Could you check this over
Fragment of a discussion from User talk:Rua
Hi. I'm pretty sure that *weh₁i- would become *wei- in Celtic, not #wēi-. Beekes makes the exact same point about the sequence *-eh₂i- turning (generally) into *-ai- not #-āi-; *-āi- must reflect *-eh₂ei- or similar.
Ok, but ei generally becomes ē in Celtic, so does a former eh₁i also become ē or does it remain separate?
Also, the shortening of long diphthongs is because of w:Osthoff's law.