The qualitative study referred to here investigated what a class of undergraduate pre-service music teachers could learn from conducting a piece of action research for primary 5 pupils in Hong Kong. Data were collected from lesson observations, post-lesson conferences with the pre-service teachers, their presentations and individual reflection reports. This action research helped the pre-service teachers to design their research lessons according to the good practices of "teaching students to understand music" and "engaging pupils to learn music through musical activities". Informed by the research data, a teaching gap was found and the pre-service teachers started to reevaluate how the pupils learn, and to conduct deeper reflections on their own teaching effectiveness. The study concluded with a rethinking of the practice of teaching the musical concepts that are prevalent in music classrooms.