The origin of the Slavic 2 sg. present ending
The two 2 sg. present ending variants of Slavic, -sь/-šь and -si/-ši, are derived from pre-Slavic *-si and *-s-ai̯ s, respectively. The first variant is found in all Slavic languages, and in most of them it is the only one. The second variant survived mainly in Old Russian (cf. neseši) and Novgorod (cf. ideši) and as a relic form in other languages (cf. Ukr. jesý, Cz. jsi, SCr. jèsi). In both variants, the form with -š- is dominant. The first variant goes back to the Indo-European primary ending 2 sg. *-si. The second variant reflects the Proto-Slavic form of the 2 sg. future with the sigmatic stem*-s-ai̯ s. This complex formant was formed in the Proto-Slavic period by adding the ending of the imperative 2 sg. *-ai̯ s (from the Baltic-Slavic imp. *-ai̯ s that continues PIE opt.* -o-i̯ h₁-s , compare OCS ved-i , OPr. wed-ais ) to the ending of the future 2 sg. * -s (from PIE * -s-si ; the athematic sigmatic stem of the future is reconstructed for Balto-Slavic on the basis of the Eastern Baltic languages, cf. Lith. duõs , Latv. duôs ‘he will give’). A partial apocope of the final * -i in Balto-Slavic could be the main reason for this morphological innovation. As a result of the apocope, a formal contrast developed between the forms with accented ending (2 sg. * u̯ ede-sı̍ ‘you lead’) and with unaccented endings (2 sg. * de̍ d-s ‘you put’). Τhe reduced ending * -s merged with the sigmatic future stem, which provoked the renewal of this form with the imperative ending*-ai̯ s in Balto-Slavic or Proto-Slavic, cf. *dōd-s-si > *dōd-s-s > *dōd-s → *dōd-s-ai̯ s. The loss of the sigmatic future and the reduction of *-s in one of the two allomorphs of the 2 sg. present form conditioned the creation of the ending *-sai̯ s and its spread to the paradigm of the present.The dominant forms -ši and -šь resulted from the first palatalization of velars from* -xi and * -xь , which in turn developed from * -si and * -sь according to the RUKI-rule in present stems in -i- . The original forms with * -s- were preserved only in athematic root present stems.