The aim of this paper is to determine the level of knowledge that younger pupils have concerning phonologically conditioned alternations. At the beginning, the paper offers an overview of alternations of the Croatian language with regard to their conditionality, and discusses the connection between the alternations, orthography and orthographic norm. Possible spelling solutions that current orthographies offer are provided, and the distribution of orthographic topics among grades according to the Curriculum (2006) and the Curriculum for the subject Croatian Language (2019) is presented. The main objective was determining the level of knowledge of phonologically conditioned alternations based on research on the use of the orthographic norm in younger pupils. Moreover, the mentioned research tried to ascertain how students write down words in which the mentioned alternations occur when they hear them and when the words are shown through pictures. In addition to words from Croatian language textbooks, words from textbooks in other subjects (mathematics and Science and Society) were included in the research. In addition, lesser-known words that students encountered for the first time were also included. Therefore, an attempt was made to determine the difference in the performance of writing the two mentioned groups of words. The paper also tried to show the differences between alternations through classes and possible differences in the age and gender of pupils. The research was conducted in a school in Zagreb County and 90 boys and 96 girls participated. Pupils were required to write down a text in which the target words were presented through pictures or dictated to them. While doing so, the pupils were required to respect the orthographic norm. From the results of the research it can be concluded that although orthographic topics are represented in the teaching of students in the examined classes, their results show a low level of knowledge concering the orthographic norm, and it is concluded that orthographic rules are dealt with on the example of concrete words, without linking the content with other subjects, or putting enough focus upon them in teaching.