Comments:ОСНЯ 2, 57. A monosyllabic root, but, unlike the 1st p. pron. or the accusative particle, it did not undergo denasalization in PA. This may be explained by the fact that it was in most cases already incorporated into the verbal form as a suffix. It is interesting to note Mong. *büi, *bu 'neg. particle' - which may be originally the same morpheme, but functioning as a separate word and thus subject to the rule *mV > *bV.