* -거든 has two different usages. 1. It is used for a condition or supposition. e.g. 집에 도착하거든 전화해 주세요. (Please call me if you get home.) e.g. 너무 비싸거든 사지 마세요. (Do not buy it if it is too expensive.) 2. -거든 is used to express a reason or some background explanation for something. e.g. 제가 왜 밥을 안 먹느냐면, 요즘 다이어트를 하거든요. (The reason why I am not eating is because I am on a diet these days.) e.g. 커피를 마시면 잠을 못 자거든요. (It's because I can't sleep if I drink coffee.)