The overall aim of the present study is to develop knowledge of Swedish students’ writing in English, and how teaching of a specific kind of writing can be designed and enacted. The study focuses on what the students need to discern in order to develop a more differentiated knowledge of how to adapt a message to an unknown receiver—in this case a message for a person at a hotel. The research question addressed is how aspects of text and receiver can be varied and explored by teachers and students jointly in order to expand the students’ capability to adapt a text to an unknown receiver. The study is based on transcribed lesson data from a learning study, which is a research approach where teachers and researchers work together in an iterative process to understand and improve teaching and learning of a specific object of learning. The research question was explored in five cycles with five different groups. Thirty-four Swedish students, 14–16 years of age, from a special school for students with dyslexia and neuropsychiatric disorders, participated in the study. The theoretical framework of the study was variation theory. A basic assumption of variation theory is that, in order to develop a certain piece of knowledge, it is critical to discern some particular aspects of that knowledge. To enable the discernment of such aspects, they must be made discernible by means of variation. The results show that a short message, used as an example, needs to be deconstructed into its aspects. Once the students had discerned the concept of the receiver, they started to contrast ways to express the same content for known and unknown receivers. With the help of the concept of the receiver, the students explored the aspects amount of information, politeness, and formality together with the teachers. Each aspect needed to be focused on separately but within the framing whole of the specific context, that is, writing a message to a hotel. The findings also show that certain aspects on the macro-level were possible to discern when two texts were compared, whereas other aspects on the micro-level, such as modal verbs, had to be varied against the background of an invariant clause in order for the students to discern them.