8.4.1. Prepositions were not originally distinguished from Adverbs in form or meaning, but have become specialized in use. They developed comparatively late in the history of language. In the early stages of the IE language the cases alone were probably sufficient to indicate the sense, but, as the force of the case-endings weakened, adverbs were used for greater precision. These adverbs, from their habitual association with particular cases, became Prepositions; but many retained also their independent function as adverbs.
Most prepositions are true case-forms: as the comparatives ekstero (cf. external), ndhero (cf. inferior), supero, and the accusatives kirkom, koram, etc.
8.4.2. Prepositions are regularly used either with the Accusative or with the Oblique.