The Hittite suffix corresponding to Indo-European nā forms verbs of the hi -conjugation, which corresponds to the IE perfect system. Phonetic law induced in IE a confusion with the neu- verbs in the first plural and another phonetic law produced a first sing, that might be interpreted as belonging to the present system. Analogy then completed the transfer of the entire category to the present system. The origin of the Hitt. suffix sa is unknown, and the suffix a may have originated in Hitt. itself.
E. H. Sturtevant (Tue,) studied this question.