The book analyzes the initial stages of a child's development of his native (Russian) language. The main focus is on the formation of the grammatical component of the language system, which consists in mastering morphological units and rules. Formative and word-forming innovations are considered as evidence that the child independently constructs his own language system, adhering to a certain strategy. Analysis of the data of this strategy allows us to obtain some new information about the internal structure of the language, about the nature of morphological and word-forming categories, about the ratio in the language of prototypical and non-prototypical units and constructions, about the prohibitions and restrictions imposed on the effect of language rules, about the ratio of reflective and interpretive principles in the language, etc.