This study examined how teachers of Korean as a second language in the U.S. adapted to online teaching during the pandemic. At a Korean Heritage language school located in the US, qualitative data including survey (n=11) and follow-up interview (n=8) of teachers were collected and analyzed. Teachers shared their own perceptions and practice on using target language only (Korean) or using English in their classrooms. In the preliminary results, this study shows the dynamics of online teaching depending on different age group and the proficiency level of Korean. Especially, depending on the grade level of students, teachers need to use different strategies depending on their motivation and interests in learning Korean. With teaching experiences, teachers showed different use of languages. For older students, using English was beneficial while parents’ support was helpful for younger students. Findings indicate that learner variables such as age, proficiency, and language use affected teachers' decisions.