Conceptual grammatical knowledge is an area which causes problems at different levels of education. This article examines the ideas of conceptual change theory as a basis for establishing a new grammar teaching method. The research strategy which I use is educational design research and the research data have been collected from teacher students in two Finnish universities in 2011-2013. When learning the concepts of grammar, problems appear in the form of reduced definitions of concepts and misuse or mixing of different grammatical categories. The languaging and visualisation method induces learners to make the meanings clear and express their grammatical thinking through different modes: speaking, writing and drawing. The categories of grammatical concepts are made visible by charts and hierarchies, which helps students to apprehend the ontological differences between the categories. By following students' languaging processes, a teacher gains a deeper understanding of their conceptual grammatical knowledge.