The article presents an analysis of the historic evolution of the subordinate conjunction "ki". It is underscored that modern linguists usually point to the mostly spread function of this conjunction - that of subordination to the main sentence without touching upon other functions of the given polyfunctional auxiliary element. The author makes an endeavour to prove that the conjunction in question performed other functions in the Tajik language as well. Adducing numerous convincing examples From Old Middle and New Persian languages it is proved that the conjunction"ku" performed the functions of conjunction (ku), interrogative pronoun (ки) and ( ки) had different forms and in the Middle period of the development of the Persian-Tajik language it acquired the form "ke".It is established that the conjunction in question is a derivative from the interrogative pronoun "ke" which superseded synonymical conjunctions in the process of a further evolution of the language.
Key words: history of the Tajik-Persian language, conjunction, language evolution poly functional auxiliary elements, functions of subordinate conjunction "ki", monuments of Old, Middle and New Persian languages
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