In the existential sentences of Slavonic languages we can find some interesting deviations from the basic type of Indo-European sentences, ie. "Nominative + concordant Verb", for instance Genitive of negation; in some, especially South Slavonic languages there are examples of the main nominal part of positive existential sentence (ie. name of the existing entity) in Genitive or even (as in Slovenian povsod jo je) in Accusative. These deviations can be of interest for the study of the development of Indo-European syntax, as Miklosich and Potebnya already in the 19th century observed. Relevant in this aspect also is the opposition between autosemantic (existential or possessive) esse and (zero or non zero) copula. This phenomena are here studied from the standpoint of the general opposition between polymorphic and monomorphic structures of the syntactical system.