This study aims to discuss how inquiry should be carried out in grammar education, especially as regards phonology. Inquiry is the most important concept for grammar education. It indicates not only a teacher’s method or skill but also their educational philosophy, which affect the selection of educational content. In this study, grammar, which is the object of inquiry, is defined in terms of principles rather than rules. Accordingly, it makes it possible for grammar education to include content about language performance such as language refinement or code-switching, which are already accepted as grammar education content. Therefore, the author emphasizes the function of phonological learning content as an aid to communication, especially in pronunciation and spelling, and also illustrates some educational activities.