must not believeUsed to say that someone should not believe something. 안 돼 means that something is not allowed or should not be done; 믿어선 안 돼 is a shortened form of 믿어서는 안 돼.
KO: 인터넷에서 본 모든 것을 믿으면 안 돼.
EN: You must not believe everything you see on the internet.