The origins of the preterite of the Old Irish copula and substantive verb: an overview and new ideas

As is well known, Old Irish presented a morphological and functional distinction between the copula and the so-called 'substantive verb'. While in the present indicative the former is based on the PIE root ^*h1 es- and the latter on PIE ^* steh₂^-, all other tenses and moods of both verbs are formed from the PIE root ^* bhuH-. Although these forms have often attracted the attention of scholars, several details of their prehistory are still unclear. This article will focus on the origins of the preterital forms: in particular, a solution for the striking difference of vocalism between the 3rd singular of the substantive verb (boí) and the copula (absolute ba, conjunct -bo, -bu) will be proposed. It will also be shown that the answer to this specific problem can shed some light on the Irish differentiation of the two forms of the verb 'to be', a process which, as will be suggested, may depend on the same mechanism which brought about the long-debated distinction between absolute and conjunct flexion in Old Irish.