As ONCE is told as the beginning of a story...a story told from word of ear and taken to be true... ONCED would be a story told from a personal account as the one who experienced it first hand.

What is the plural of once?

Noun. once f (plural onces)

What is the adverb of once a week?

Weekly most commonly means once a week or every week. It can be used this way as an adjective, as in a weekly meeting, or an adverb, as in We plan to meet weekly. Something described as weekly may happen on the same day every week or simply once a week at any time.