In this article, we present a case study about a teaching practice of the genre “reply to the reader's letter” directed to students aged 9 to 10 years of a primary school from the canton of Valais, in Switzerland. We analyzed the learning obstacles emerged in the implementation of a didactic sequence about this genre and the teacher's actions in order to overcome them. We grounded our study theoretically and methodologically in the studies of Bachelard (1938/1993) and Brousseau (1998) for the definition and initial identification of the obstacles and in the studies developed by Schneuwly and Dolz (2009), Silva (2013) and Silva-Hardmeyer (2015) and Tobola Couchepin (2017) for the analysis of the teacher's interventions. The results of the analyzes show that overcoming the learning obstacles is associated with the types of the teacher's interventions, anticipating possible difficulties.