So I was told that the English articles "a" and "an" have Germanic origins. In German, there is not a distinction between "one" and "a/an". Is there any explanation for the existence of these two different words in English? Is there a situation in which "one" and "a/an" cannot be interchanged? I suspect that "a/an" is just easier to say....but maybe I am just biased to think so, as I am a native English speaker.
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).