in the past과거 means “the past,” and 에 marks the time or topic being referred to. The particle may change or be omitted depending on the sentence, as in 과거에는 (“in the past” with emphasis) and 과거에도 (“also in the past”).
KO: 과거에 이런 일이 한 번 있었어요.
EN: Something like this happened once in the past.