The paper reports a study of several Russian syntactic idioms in which one of the lexical units is čto (a subordinating conjunction or relative pronoun): The study is focused on syntactic and combinatorial properties of these linguistic entities and their meaning. The issues of adequate treatment of these entities in natural language processing tasks are discussed. The main difficulty here is reliable identification of these entities which could distinguish them from cases where the constituent elements of the idioms appear in the text independently of one another. A case study of one polysemantic syntactic idioms is presented - tol’ko čto ‘just; only’ in contradistinction to the polyseomous particle tol’ko ‘just’.