The article touches upon the problems of general theory and metalaguage of one more invariant type of the sentence, that is, a composite sentence with an introductory clause along with the three traditionally accepted structural-semantic (compound, complex and mixed /semi-composite/) types and the taxonomy of the latter in language. The main focus here is made on the so called parenthetical, rather introductory constructions represented by introductory words, phrases and sentences, specifically verbalizing the universal concept of subjective modal assessment” of the fact (action, quality, quantity, state, etc.) expressed in the main body of the sentences, to which the latter are canonically introduced. The author here makes an accent on the status of the so called simple sentences with an “introductory element”, expressed by a clause in language(s) which have been traditionally treated as “simple sentences of the complicated structure”, although they are characterized by their natural surface and deep structures like a composite sentence with at least two predicative units, each of which having its own grammatically expressed subjectpredicate structure. Critically approaching the treatment of such composite sentences as “complex sentences with a parenthetical clause” in special literature, the author considers them to be “composite sentences with an introductory clause”, for the latter can’t be segmented into principle and subordinate clauses, and proves his own approach by illustrating them through convincing examples from fiction materials, which paves the way to four–membered taxonomy of composite sentences replacing the traditional three –membered one.