The purpose of this study is to investigate how the teaching of mathematics is reflected in the students' understanding of the concept of the subtraction. The empirical data is based on observations in the classroom and student interviews. The understanding of the concept of the subtraction is shown through vocabulary and choice of examples and representations. The study used both qualitative methods and quantitative reports. The data is represented by one school and one classroom in grade three.  The theoretical framework is based on Liping Mas Subject Matter Knowledge and the variation theory of Marton and Booth and the relationship between these frameworks and concepts of variation, concretization, aids, subtraction and types of subtraction.  The results show that the impact of education is that subtraction is understood as the invers of addition. Complement and add to, is the type of strategy most frequently used, followed closely by the strategy remove and count down. The strategy compare is the least used. The observations in the classrooms shows several aspects and variations of the concept of the subtraction. Although the number line, and various forms of representation used in teaching has shown small effects of these aids in the students' own counting. The conclusions that can be drawn is that more and other metaphors and tools are needed in education to deepen students' understanding of the concept of the subtraction. They also need to be connected in the teaching. The language and the mathematical ideas discussed plays a critical role in the pupils’ acquisition of understanding.