This qualitative research presents how a mathematics teacher educator's feedbacks to two lower secondary school mathematics teachers affects their design of hypothetical learning trajectories regarding teaching patterns and their teaching practice. The data comes from a web portal, which was designed as a professional development tool for mathematics teachers. The teachers communicated with a mathematics teacher educator online, and prepared lesson plans, as well as receiving feedback. After implementing their designs, they also uploaded teaching episodes to the web portal, after which they received feedback. We triangulated data that came from the lesson plans, mathematics teacher educator's feedback and videos of teaching practice. The data were analysed within two lenses: the Knowledge Quartet and the Hypothetical Learning Trajectory. The findings reveal that through the feedback given by the mathematics teacher educator, the teachers substantially progressed in designing hypothetical learning trajectories and developed their professional knowledge, in particular regarding the foundation, transformation, and connection dimensions of the Knowledge Quartet.