The pronouns это and то display great functional diversity in present-day Russian (PDR): they can function as demonstrative pronouns, copulas and subjects with verbal or nominal predicates, and adverbial predicatives. Comparing the ways of expressing the subject in Old Russian and PDR I find that это и тo as subjects are relatively new phenomena. The article aims at exploring the diachronic reasons behind this change. In doing so, I first take a look at the transformation of the Old Russian

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... em of demonstratives and the formal evolution of the pronouns это и тo. Next I investigate parallel syntactic processes (use of 3rd personal pronouns in subject function, appearance of -o final adverbial predicatives, spread of infinitival subjects with это) that probably facilitated the functional expansion of these pronouns.