The article is about the observation of the interfering influence of the Italian language on the formation of plural forms in the Russian speech of Italians, which can lead to the erroneous formation of plural forms from the Russian words singularia tantum. The purpose of this study is to establish the main linguistic causes existing in the Russian and Italian languages, which are the basis for the occurrence or absence of such type of interference and related errors; which, in turn, allows us to understand the nature of errors in the speech of non-native speakers. The research was conducted on the material of authentic translated texts written by Italians for educational and other purposes. The relevance of the topic is due, on the one hand, to the frequency of the formation of plural forms from the words singularia tantum in the Russian speech of Italians; on the other hand, the need for a clear understanding of the linguistic nature of such errors. It is important to emphasize the unsystematic nature of the formation of number forms in nouns in Russian and Italian, as well as the unsystematic nature of grammatical litters describing the category of numbers in bilingual and explanatory dictionaries, which creates a significant methodological difficulty.