According to the activity theory, design students plan, realize and describe the tasks they have to perform. This process is a specific way of expressing their design learning process, within the curriculum in which they are interacting. The curriculum contents are discussed within the Pedagogical Contents Knowledge (PCK) framework to improve the teachers' methods. Sustainable development (SD) should be treated as a priority in the specific Technological Pedagogical Contents Knowledge (TPACK) research, since SD is a technical and social issue. In this paper we study the methods used in two Masters of Arts courses to teach SD. The design students' statements are psychologically and semiotically analysed. Examination of the students' verbal and non-verbal utterances or iconic and non-iconic signs show hesitant, diverse and weak-structured statements. Numerous indecisions emphasize their contradictory conceptions of SD. The lack of SD specifications in the syllabi opens a wide space of discursive perceptivity, in which students build their own design idiolect. They acquire design skills and abilities challenging the design PCK. In this way, we can use these skills and the students' understanding of SD to develop the TPACK. In comparison to previous studies on SD learning, this paper emphasizes the need for combining these approaches to structure 'design didactics'.