Well, we've kind of been through that when talking about the definite article. The only thing that doesn't apply is that there's an abstract, mathematical number "eins", similarly to "uno" in Spanish - I know this isn't particularly useful to you, but I actually wasn't sure if this wasn't a freak, isolated phenomenon. Because in all the other languages (that I know) the mathematical number is the same as calling the number of things: "un" in French, "één" in Dutch, "one" in English, of course.

But in Spanish there's "uno, dos, tres" and "un niño". Just as in German - "eins, zwei, drei" and "ein Junge".

The rest of your problem - well, things of course have to be inflected in German, as opposed to English. And there's no clear separation between the indefinite article and the... cardinal adjective "ein" in German. But in English "a" has the same root as "one".