"einen" is just the accusative of "man" in this context. (it is not an article and it has nothing to do with the number 1)
You do not have this word at all in English, and often a good way of translating "man" is "you"
"Children change you!"
Declination of "man" :
Nom. "man" - Man weiß das nicht recht.
Gen. - (a genitive of "man" does not exist)
Dat. "einem" - Sowas muß einem doch erklärt werden!
Acc. "einen" - Das kann einen schon sehr deprimieren.
(Not a native) 'Man', strictly speaking, exists only in nominative. In the other cases, it is replaced by the forms of the indefinite pronoun 'eine(r,s)'. To my mind, 'man' and 'eine(r,s)' have basically the same meaning, the only difference being that 'man' always refers to an undetermined person, while 'eine(r,s)' can also refer to an undetermined person/thing that has already been mentioned (a sort of anaphoric use). For more info and examples, visit the page: https://goo.gl/YgEvrI
I tend to think of "man" as referring collectively to humans (e.g. mankind) and not just someone in particular. That is one way to look at it, and it helps to differentiate the word from "ein". It certainly helps me since if used as an object in theory, I might think of it as not just being one person, but all of mankind.
That may or may not be the accurate definition of the German "man", but that helps me in avoiding the confusion.
English does have this word!
My understanding is "Man" (not to be confused with Mann) = One = singular form of Mankind.
In this context one can means formally:
1) "a" man / "a" person / "a" human being / someone / I / you / he / she / they
Einen (akk) is just a contraction of "einen Man"/"a man"/"you" to simply "a".
In English "a man / a person / a human being / someone / I / you / he / she / they" is contracted to simply be "one". It would not be contracted further.
Only similar contractions to the German I know are things like "Thee" for you/yourself" etc. but they aren’t really used in modern English but "one" is still used a lot in certain circles and contexts as far as I know.
E.g. One shall not speak in the library.
One should not think of such things.
One means you no harm.
Children change one / Children change a person / Having children changes someone.
Although one must admit, when one should speak in such mannerisms, into the ear of one another, could lead one to appear a fraction pretentious!
Of course we have the word in English. I first learned to translate "man" as "you" in the sense of "one", which was totally familiar to me. Now "one" comes to my mind every time I see "man" and that helps me immensely. This is the first time that I am learning that German uses "one" in the same context that English does. Now I have to learn to decline it. My vocabulary is growing, but my grasp of grammar is lagging far behind. I need to go live in Germany for a while.
Here's an interesting fact. In English, "one" can even refer to the first person. "One thanks the gentleman for the lesson." One (third person meant here) might use that sentence to parody how a "proper" butler speaks.
Yeah you are right, Copernicus, apologies, got my wires crossed, I kinda thought that after I posted but hoped that nobody would read my post. I blame my age for the confusion, hope I didn’t confuse anyone else! But you must admit that using ‘one’ in English is quite royal.
A really good explanation between ändern and verändern can be found here https://german.stackexchange.com/questions/5898/what-is-the-difference-between-%C3%A4ndern-and-ver%C3%A4ndern
Duo's translation is 'Children change you'. I think that there could be a few different/better ways to convey that in German. Einer would not be my first choice. What about dir, euch, Ihnen? Children change a person or Children change one are different than children change you. C'mon Duo, clean it up.