This paper aims to investigate the interrelation between aspect and modality in Russian. For this purpose we analyze the effect of the use of aspect in irreal" conditional sentences and imperative sentences, which can be counted as the ideal types of marked moods in Russian. Conditional sentences are designed to show (sequential and/or logical) causal relation between the two propositions, i.e. between the proposition in the protasis and the proposition in the apodosis. Since causal relation often implies temporal sequentiality, temporal sequentiality, in general, is highly correlated with perfective aspect. However, when the speaker has an intention to weaken the assertion of causality, he may substitute imperfective aspect for perfective. In imperative sentences where interpersonal function of communication is activated, substitution of imperfective aspect for perfective also assumes the function of weakening the demand for the termination of action. In imperative sentences the weakening effect minimizes the distance between the speaker and the listener and serves as the means of expressing speaker's intimacy and politeness toward the hearer. We can conclude that in both sentence types speaker's pragmatic intention serves as a motivation for the aspectual change
