The article analyses stages in the formation of the complex sentence in terms of studying the process of gradual linking of a growing number of phrasal units to the simple sentence. The subordinate clauses proper – the object, adjective and adverb clauses – are treated as the final stage in the process of complication when two relatively autonomous units are juxtaposed to one another. This final stage is preceded by two preliminary degrees of complication when the separate parts stand in correspondence to the main sentence. The article focuses on the first degree of complication which deals with the so-called identification system consisting of two groups and four sub-systems depending on the expanding and specifying of the content of identifiers.