Origin of distinction between "one" and "a/an"?
A/an isn't used when counting. What's more, in narrations, we say one day, one afternoon, one cannot be replaced by a/an in these expressions.
It's true that in other languages the indefinite article isn't different from the word denoting the number 1. In The Free Dictionary it is stated that the differentiation came about through the pronunciation of the word one without stress (I suppose when not counting).