In this article we are going to trace the transition of modal verbs into epistemic adverbs. This development found in nearly all European languages will be analysed on the basis of Russian možet (byt’) which goes back to the third person singular present tense of the modal verb moč’ plus infinitive. In our study we shall focus on the constructional aspects of this language change with the aim to show the close interaction of semantics on the one, and argument structure on the other hand. The first part of the contribution is dedicated to a synchronic account and the second shows the diachronic development of the constructions involved. We will argue that the rise of epistemic sentence adverbs involves a third construction traditionally called complex subject sentence (možet byt’, čto p). It will be shown that the rise of sentence adverbs is a complex process consisting of both gradual and discrete micro-processes.