The task of creating innovative technologies for teaching Russian as a native, as a non-native and as a foreign language, taking into account the age of students, their cognitive abilities and features of the first language, is relevant. The solution of this problem requires a fundamental psycholinguistic study of the processes accompanying the formation of the language mechanisms of children and adults. The aim of this research is to specify the ideas about mastering the category of Russian case in various conditions of language acquisition. The method of comparative analysis of the processes of mastering Russian case in monolingualism, simultaneous child bilingualism, studying Russian as a foreign language and Russian language acquisition by a foreign-speaking child in the process of communication, was used. The material was collected during longitudinal observation of the speech of primary school students from St. Petersburg, who were native speakers of Turkic and Caucasian languages. The classification of errors in selecting and constructing case forms is proposed. The ways of access to the mental lexicon and the mechanisms of procedural grammar rules formation are described. The conclusion is that there is the imbalance between the cognitive and communicative development of a foreign child and the level of language proficiency, which leads to a detailed, but grammatically unformed statement, characteristic of a successive natural bilingual. When Russian-speaking child resorts to retrieving ready-made frameworks of prepositional case constructions from memory, the foreign child independently constructs them in speech, preferring to use basic and reduced forms. In the future, it is necessary to develop a unified toolkit for studying grammar mechanisms formation and to increase the empirical base of the study. The information about speech ontogenesis obtained as a result of the study must be taken into account for developing scientifically based methods of teaching Russian as a non-native language and as a foreign language.