What always nagged my curiosity was why on earth Proto-Indo-European (PIE) had a first person singular in *-oh₂ for thematic present-future forms (e.g. *bʰér-oh₂ "I carry") yet *-mi for athematic ones (e.g. *káp-mi "I take"). In the past I came up with a whole bunch of completely lame excuses, but I assure you all I was desperate in the absence of guidance from wiser academics who didn't seem to touch the subject in a comprehensive and comprehensable way. However, I think, my dear Watsons, I've finally got it!
It seems to me that the first person of the subjunctive implies something that is different from all other persons. In most languages, the subjunctive is the mood we use when we want to indicate a hunch, emotional feelings, a hypothetical situation or any other idea that lies outside the strict statements of empirical fact that the indicative is meant to cover. Naturally, however when we convey our own feelings or our own potential acts in the first person, we are surely speaking with the utmost authority. Afterall, who else knows best what I will do but I myself. (Speaking of which, I will no doubt go to a local café again to ponder on early Indo-European grammar some more, hehe.) The other persons of the subjunctive are different because we can never be as sure what the potentiality of an action or the true feelings of another person really are, were or will be. Thus we can say that the first person subjunctive is a more certain statement of irrealis when compared to all other persons.
With that revelation, let's agree then that original PIE (the one that preceded the oldest divergeance, that of the Anatolian and Tocharian languages) likely contained the following conjugated forms:
- indicative present-future 3ps *bʰēr-ti "he carries"
(later *bʰér-e-ti via the subjunctive)
- indicative preterite 3ps *bʰēr-t "he carried"
- subjunctive 3ps *bʰér-e-t "he would carry"
- mediopassive 3ps *bʰer-ór "he carries (something for himself)"
(later *bʰer-e-tór or *bʰer-e-tói)
Now we come back to the 1ps of the subjunctive which low and behold seems to contain a perfective ending, something that I contend marked the reflexivity inherent in the subjunctive of that person. When stating, for example, "I would go", one is in effect saying "I would go (speaking for myself personally)". It's no wonder then, considering it this way that we should find the subjunctive 1ps terminating in *-oh₂ (that is, that ubiquitous thematic vowel *-e/o- plus the old perfective ending *-h₂(e) that marked the ancient reflexive as it existed before the adoption of a specialized mediopassive conjugation).
Now I can make sense of the suppletive pattern we see in the subjunctive-turned-future-indicative endings:
- 1ps *bʰér-o-h₂ "I would carry (speaking for myself)"
(later *bʰér-o-h₂ "I will carry")
- 2ps *bʰér-e-s "you would carry (as far as I know)"
(later *bʰér-e-si "you will carry")
- 3ps *bʰér-e-t "he would carry (as far as I know)"
(later *bʰér-e-ti "he will carry")
 The above thought, by the way, is built on ideas already published (see Hittite and the Indo-European Verb, by Jay H Jasanoff) but I'm just not sure whether anybody explained the subjunctive in this way before. Hmmm...
 Some people, I wager, will be still confused about why there is no *-i on the later thematic 1ps indicative *bʰér-o-h₂ but in order to explain this we need to appeal to Mandarin Chinese and its particle zài as a real-world parallel of both grammar and etymology with that of the marker *-i in Proto-Indo-European. Just as Mandarin zài, a particle literally meaning "being there" used to mark continuous actions, is barred from perfective forms with particle le, so too would *-i be likewise disallowed in PIE's perfective aspect, despite many IEists who like to entertain the possibility of the two endings coexisting together. If what I'm saying is correct (that the 1ps subjunctive was in effect an old reflexive perfective), then we have a grammatical reason to justify the attested avoidance of *-i in being used in *-o-h₂.
(Aug 3 2007) I don't know what possessed me to put the accent on the thematic vowel of the subjunctive paradigm (i.e. I wrote **bherét instead of proper *bhéret) but I took my medication and I feel better now. The accent should be on the root and so I've changed all instances of it here. Good thing I review my own crap. In fact, I can't explain the alternating *e/*o pattern of the thematic vowel unless it is unaccented so this suits me just fine. I plan to talk about thematic vowels further in an upcoming blog entry.
(Mar 19 2008) I corrected the following "In its earliest form, it's known to have used the same endings as in the perfective but with the addition of a mediopassive suffix *-r and a shift of the tonal accent to the penultimate syllable." The part in italic bold was changed to "suffix". I assume I was mixing up different stages of Pre-IE when I wrote this because this statement is only valid for Mid IE.