The article actualizes the problems of grammatically correct speech formation in preschool children. The conditional standard of norm of mastering by the child of preschool age of a native language is revealed.With the help of longitudinal and scientific research of well-known Ukrainian and foreign authors on the formation of grammatically correct speech in preschool children, which cover all ages and the whole grammatical structure of speech (morphology, word formation, syntax) the current results and valuable experience of the process of speech formation of children of early and preschool age are highlighted. The appropriate use of the term «genesis» of the grammatical structure of the language of a preschool child is proposed, which refers to the appearance, development and formation of this process in children. The so-called «free» or unstimulated speech of preschoolers, depleted in their vocabulary and grammatical forms, is mentioned. The importance of controlling a child's speech by an adult and the use of effective means of stimulating is emphasized. Attention is paid to the problem of formation of grammatical structure of language in preschoolers, which is relevant for adults to understand for the development of children's speech. An attempt is made to generalize the genesis of children's speech and identify stages of speech and speech development of preschool children. The results of the author's researches of children on the material of stimulated and unstimulated speech on the example of adjective adverbs are demonstrated. The importance of further study of the functional aspect of the formation of grammatically correct speech and the disclosure of language and speech potentials of children is emphasized.