The purpose of this study is to analyze pre-service teachers’ knowledge base in terms of their substantive subject matter knowledge and syntactic subject matter knowledge. The data collection includes pre-service teachers’ reflections upon their written analysis of teaching observations as mathematical content and written examples of how to implement a teaching moment (content) that they had created themselves. The crucial content for the study is algebraic patterns, rational numbers (fractions), proportionality, and combinatorics. The findings from the study identify and highlight areas to be developed, especially with respect to student teachers’ knowledge base and what they need to explore concerning subject-specific content for teaching mathematics, as well as crucial relevant content (substantive knowledge) and how to apply this in teaching (syntactic knowledge). Generally speaking, the study highlights the challenges student teachers face in grappling with mathematical concepts when they describe their observations, as well as in their own construction of a teaching moment. This means that in teacher education for mathematics, more attention ought to be paid to developing a knowledge base founded on the transformation of substantive knowledge to syntactical knowledge and its impact on student teachers’ learning of mathematics for teaching purposes. The study can also provide a deeper understanding of student teachers’ learning process and challenges related to the knowledge base, e.g., its substantive and syntactic components.