"He is a child."
Translation:Han är ett barn.
There are two articles: "en," which is common gender (all the formerly masculine and feminine together), and "ett" (or "et"), which is neuter. "Barn" is neuter and so takes "ett." By comparison, "flicka" ("girl") and "pojke" ("boy") are both common and so take "en." We just gotta memorize which goes with which! That's all I know so far. Hope it helps!
There is really only one way to do that: One memorizes that "barn" is neuter and therefore takes "ett."
One thing that one can try is to use some little trick to memorize a word's gender--some way that one can remember that "child" is neuter.
Then after that, I find that I get used to hearing "ett barn," or "der Spiegel" (German for "the mirror," masculine). And that word combination becomes more and more familiar until "it just sounds right." And it becomes automatic.
I hope that helps!