'Once upon a time' I was a princess but then I kissed the frog!

Some students like to use the phrase, "Once upon a time" to talk about a real event that happened to them in the past........biiiiiiiig GHALAT. Once upon a time is like saying in Arabic, kan ya makan...

'Once upon a time' is reserved for fairy tales, children stories like Cinderella, Snow White, El Shater Hassan, Ali Baba, etc...
Lou inta Ali Baba or el Shater Hassan, mashee...ib'a oul Once upon a time I lived in a small house in the forest. Bas lou inta banee adam wa 3andak shahadit milad zayena keda, ma yinfa3sh testakhdim once upon a time lil madee. Wala tinfa3 tistakhdimha for historical facts (haqaiq tareekheya).
Eh? han oul: Once upon a time there was World War II (????). mish hadouta heya!

The word 'once' is enough to describe something that you did one time or at one period in your life. You can use it at the beginning, middle or end of a sentence, depending on the meaning you want to give:

- I once went to the circus. (mara roht el serk) (nifham keda inaha ghalibin mara min el marat ili roht.) We use this format especially when we want to express that something else happened after the first action.
I once went to the circus and met a clown...meen? gozee?! habibi!!

-I went once to the circus. (roht mara lil serk) (same meaning as above)
- I went to the circus once. (roht el serk mara be ma3na mara wahda fee hayatee).

ya3nee the point is, 'once' is used to express a one time action in the past.

3ala fekra, you can use 'once upon a time' when you are making fun of something...tary'a ya3nee:

Once upon a time - before I got married - I was a princess living in a big castle. Life was great! Then I kissed the frog and he became 'a' husband and ..ehhhhhhhhh...we got married. I guess I made a big mistake....once!