The article deals with the issue of expressing the category of certainty-uncertainty in the opposition of accusative and genitive cases. The dependence of the reasons for the choice of accusative or genitive case on the meaning of certainty-uncertainty is described. The category of certainty-uncertainty is considered by the example of Russian texts and their translations into Persian; the examples of the expression of this category in the Russian language and their correlates in the Persian language are presented. The authors carry out the comparative analysis of the examples and their translations, find out the similarities, differences and shades of meaning in the expression of certainty-uncertainty in the Russian and Persian languages.