and I should probably ask what the heck o-grade and zero-grade and all that's about
I've been meaning to post about this, and this is a good time. A while back IE philologists noticed that the roots they were reconstructing were basically of the form TVRT where T stands for most stops (p, b, t, d, k, g, aspiracted and not), V is a vowel, R is roughly an 'l' or an 'r' (also maybe 'w' and 'y'). The vowel that usually was reconstructed was e, but o appeared often enough, too. Long story short, it was hypothesized (and not all agree on this) that e varied with o and also zero (i.e., no vowel) depending on the accent (which may have been a pitch accent, or tone, like in Chinese or Lithuanian). There were some 'a's too and long version of some of the vowels, but these were ascribed to fricatives called laryngeals (roughly and probably glottal stop, 'h', 'ch' as in German, and maybe 'gh' (voiced) as in Arabic.
So, there are these grades that IEists talk about. Anyway, it helps to regularize some of the irregularities in the reconstructed roots.
