This study employs the methodological framework of CA-for-SLA to identify developmental changes in second language learners’ competence in the use of the Korean sentence-ending suffix canh as an interaction-al resource by examining cross-sectional Korean classroom data. A microanalysis of 68 hours of video-recorded data from advanced- and intermediate-level Korean language classroom interaction focused on stu-dents’ varying degrees of competency in the production of utterances with canh to manage shared knowledge in the action of giving accounts. The analysis revealed that the two proficiency levels’ turn construction and design of canh usage are distinctive in terms of their presentation of different types of knowledge. Learners acquire the ability to use canh to manage first-hand, shared knowledge before they acquire the ability to man-age common sense knowledge and unshared knowledge using canh. This comparison of the two proficiency levels’ use of contingent methods of using the target suffix provides evidence for the development of L2 in-teractional competence by showing that learners develop the skill to use interactional devices to achieve deli-cate interactional outcomes.