The article examines the homonymy of words of different parts of speech in the context of teaching orthography to schoolchildren. This phenomenon requires special attention because, firstly, it underlies the understanding of morphology as a branch of linguistics. Secondly, homonymy is one of the causes of spelling errors which are difficult to avoid. Thirdly, understanding the nature of this phenomenon facilitates the perception of language as a system. The paper discusses the activity-based approach to learning the Russian language. Within its framework, no ready-made rules are imposed on schoolchildren. Instead, they are given problems. Solving them contributes to the development of students’ critical thinking, as well as a deeper understanding and appropriation of independently discovered knowledge. On the one hand, the study is based on the examination of linguistic units, on the other – on the analysis of students’ activity when solving orthographic problems. Based on the experience of using the Elkonin-Davydov developmental education system, we propose a tentative structure of a lesson included in the system of lessons on the topic Homonymy of words of different parts of speech. The article also describes individual assignments which are of a research nature and contribute to the development of a few competencies that transform a schoolchild from a recipient of information into a student-researcher. The element common to these lessons is an increasingly complex algorithm for distinguishing homonyms which belong to different parts of speech and are characterised by differences in spelling. The key con[1]clusions of the study reflected in the article include a description of the activity-based approach principles in relation to the study of grammatical homonymy. Motivation built on a "success situation" and a "gap situation", the use of quasi-research techniques, reliance on an algorithm for solving problems, and step-by-step practicing of each algorithm opera[1]tion are among these principles.