Sentence examples for once upon from inspiring English sources

The phrase "once upon" is correct and can be used in written English.
It is typically used at the beginning of a fairy tale to indicate that the story takes place in the past. It is also commonly used in storytelling to create a sense of nostalgia or intrigue. Example: Once upon a time, in a faraway kingdom, there lived a brave prince who embarked on a quest to save his beloved princess from an evil sorcerer.