I can't understandLiterally, “there's no way I can understand.” The pattern -(으)ㄹ 수가 없다 means “cannot” or “it's impossible to,” and 없어 is the informal form; the phrase can express confusion, disbelief, or frustration depending on context.
KO: 왜 그런 결정을 했는지 이해할 수가 없어.
EN: I can't understand why you made that decision.