This paper is based on data from two teaching sequences in primary school that are designed using principles from the theory of didactical situations (TDS). The following research question is addressed: "What opportunities can a teaching design based on TDS give a teacher to gain insight into pupils' language use, and to use this insight to establish shared, and mathematically acceptable, knowledge in a group of primary school pupils?" Empirical data from one teaching sequence on geometrical shapes and another teaching sequence on combinatorial problems are used to answer this question. The research shows that a sharp focus on well-defined learning goals does not limit the pupils' possibilities in expressing their thoughts and ideas in their own language. The research also shows that despite clear learning goals, the teacher has rich opportunities to build on pupils' language to connect everyday and scientific language for the purpose of developing a mathematically accepted discourse.