This article deals with such constructions as «имамь + infinitive» and «имоу + infinitive» and their correlation in Old Russian texts. The constructions «имамь + инфинитив», in which the verb имѣти has a meaning of inevitability or obligation, were considered as stylistically marked bookish forms – they were used in specific contexts, were not used in the letters and the Princely statutes; there are no constructions in modern Russian dialects which can be traced to имамь-conjunctions. Ιn the Novgorod texts there are no constructions "имамь + infinitive" at all. The имоу-constructions, which were peculiar to spoken language, unlike the constructions with имамь, had no modal value. On the basis of these constructions forms of periphrastic future in the Ukrainian language and some Russian dialects have developed. Probably with the course of time the copyists ceased to understand the modal value of the infinitive constructions with имамь, имаши, and имамь-constructions began to be interpreted as the analogs of living constructions «имоу + infinitive» – especially since their forms of the 3 person plural are originally homonymous – имоуть.