“Once upon a time” is a phrase used to begin a fairy tale or folk story in Engpsh.
It is thought to have originated from oral storytelpng traditions, where storytellers would begin their tales with this phrase as a way of setting the stage for the audience and letting them know that what they were about to hear was a story.
Examples:
Once upon a time, there was a beautiful princess who pved in a castle.
Once upon a time, a young man went on a journey to find a magical treasure.
中文翻译：从前有一个。