Outsider most certainly meant the loss of verb forms compared to OCS which is considerable.I have only some notions of Russian, acquired fifty years ago. Russian has perfective and imperfective verbs. This splitting does not exist in french, nor in latin. I think that imperfective past in russian is exactly imperfect in french, perfective past in russian is what we call "passé simple" (or "passé composé" when there is an auxiliary verb).
Originally Posted by Outsider
As were several Slavic languages, particularly in the Balkans, where Latin, Greek, and Celtic languages had previously been spoken.
Maybe that's why Bulgarian doesn't have cases and has complicated verbal system.