The singular dative, accusative and the instrumental cases of the Indo-European stem “o” derive from one morpheme *-oN. Various realisations derived from the etymological common form, i.e. *-o/V+ V>*-oN + V (nothing changed), but *-oN + C >*-5 + C. Usually *-oN became the indicator of singular accusative, and *-ū – of singular dative and instrumental case
