-면 is a verb suffix that literally means "if", but is also used in situations that would call for "when" in English.
한국말 잘 하고 싶으면 열심히 공부하세요. If you want to speak Korean well, study hard!
너무 많이 먹으면 배 아플거예요 If you eat too much, your stomach will hurt.
Category: Korean grammar reference