once upon a time idiom
B1 ﹝用于儿童故事的开头﹞从前，很久以前
used at the beginning of children's stories to mean "a long time ago"
Once upon a time there was an ugly duckling. 从前，有一只很丑的鸭子。
（表示对过去的留恋）以往，从前
used when referring to something that happened in the past, especially when showing that you feel sorry that it no longer happens
Once upon a time people knew the difference between right and wrong, but nowadays nobody seems to care. 从前，人们还知道是非曲直，可是如今似乎没人在乎这些了。