“Once upon a time” is the English equivalent of “原来如此” in the context of storytelling. It’s the beginning of many fairy tales and children’s stories. Here are a few variations of “once upon a time” that could be used in different contexts:
Once upon a time…
Long ago, in a land far, far away…
In the beginning…
There was a time…
Once, in a kingdom by the sea…
Each of these phrases sets the stage for a story to unfold.