This article examines the acquisition of Korean imperfective markers, the progressive "-ko iss-" and the resultative "-a iss-," with a view to understanding how tense/aspect morphology expands beyond prototype associations with inherent aspects of the verbs. We hypothesized that "-a iss-" will develop later than "-ko iss-," but that the development of "-a iss-" will precede or coincide with the expansion of -ko iss- marking for result state meaning. Cross-sectional data were collected from 120 learners of second language Korean using a sentence interpretation task and a guided picture description task. The results support our hypothesized acquisition route of imperfective markers, establishing dynamic durativity as the prototypical meaning of the Korean imperfective "-ko iss-" and suggesting individual variation in expanding the prototype.