At present there is a growing demand for teaching Russian as a foreign language due to an increase in migration to Russia and, consequently, a rise in the number of non-Russian speakers enrolled in Russian-language schools. This research analyses grammatical mistakes that non-Russian speaking children make when learning to write in Russian. The article discusses the reasons why such mistakes might occur, defines the difference between compositional mistakes and transfer mistakes, and suggests possible ways to eliminate them. The authors suggest that the research may be of interest to the teachers who have encountered such issues while teaching in multiethnic classes.