Big (2012)

Gil Da-ran (Lee Min-jung), a high school teacher, is engaged to a handsome doctor, Seo Yoon-jae (Gong Yoo). Kang Kyung-joon (Shin Won-ho) a troubled transferred student from USA, one of Da-ran's students, got into a car accident with Yoon-jae. Kyung-joon woke up in Yoon-jae body while his own body stayed in coma. Da-ran and Kyung-joon (in Yoon-jae's body) had to pretend to be in relationship while working out how to switched back.