The current research investigated the acquisition of the copula "be" in Present Simple Tense (PST) in English by native speakers of Russian. The aim of the study was to determine whether or not Russian students with different levels of English proficiency would encounter any problems while using the copula "be" in PST in English. The study also identified domains related to the use of the copula "be" that appear to be problematic for native speakers of Russian. To carry out the research, two diagnostic tests and a taxonomy of the domains of the use of the copula "be" were developed. The data were collected from three groups of Russian students who were in their first, fourth and eighth years of learning English. Quantitative and qualitative analyses were used in the study. The findings of the study revealed that some of the errors, such as omission or misuse of copula forms, disappeared with further exposure to the language. However, some of the errors were fossilised and were observed in the Russian students' performance even after eight years of learning English. Negative transfer at the morphological level and incomplete understanding and application of the copula "be" rule were suggested to "be" the underlying reasons for the fossilised errors made by the Russian learners. (Contains 4 tables.)