<p>Modelling problems, being authentic, open-ended, and complex, foster mathematics learning connected to real-world contexts. However, teachers often rely on textbook tasks and face difficulties when designing modelling problems for classroom use. This study examines how 110 pre-service primary school teachers (PSTs) engaged in problem posing by formulating modelling problems from textbook tasks. We categorised the strategies used in these formulations—aimed at enhancing authenticity, openness, and complexity—and analysed the errors found in the resulting problems. This dual focus provides a deeper understanding of how PSTs approach the challenge of transforming routine textbook tasks into modelling activities. In addition, we examined the frequency of the identified errors as an indicator of overall performance and investigated whether the mathematical content of the original tasks influenced the formulation strategies employed. Our findings highlight both promising approaches and recurring difficulties in modelling problem posing, offering valuable insights for the design of teacher education programmes that aim to develop PSTs’ competencies in designing and selecting tasks that foster students’ modelling skills.</p>