Undergraduate students fail to objectively self-assess grammar skills of English as a second language but mastering English grammar autonomously can assist. The article aims to describe and assess the Moodle plugin that offers undergraduate students individual learning paths in studying English grammar. An experiment with 3953 students was conducted at Peoples' Friendship University of Russia in two academic years: 2017-2018, 2018-2019. The Moodle plugin "RUDN CourseAssist" facilitated the English grammar study. The students subjectively stated their English language proficiency level and set academic goals in a self-assessment learner profile. The diagnostic stage of the experiment identified premises that provided individual learning paths in studying English grammar: automatic level detection (A1, B1, B2) and learning styles (active learners -- 59.9%, sensing -- 49.7%, visual -- 81%, global -- 49.7%). At the test assessment stage, 491 students demonstrated a steady rise in doing grammar exercises correctly, and performed more objective self-assessment of the level. At the plugin assessment stage, the Pearson correlation coefficient and the students' feedback in questionnaires and interviews proved that the plugin is flexible, scalable, user-friendly, and applicable. The state-of-the-art plugin to be customised for other languages is effective in providing students with individual learning paths to improve English grammar skills.