"It is a girl."
Translation:Det er en pige.
My understanding from talking to native speaker friends is that the generic "it" reverts to neuter.
I don't think this is correct. I was taught long ago that the article should match the gender. It is en pige, so den er en pige. Of course, you could also day Det er en pige. But the point is you can say it either way.
I don't understand why is "det" and not "den" because I read that det is for inanimate things and den for alive things??
"What are the two genders in Danish?" I'm afraid a short answer would only make you more confused- you need to read these two pages. http://blogs.transparent.com/danish/2011/12/08/en-or-et-the-peculiar-sex-life-of-danish-nouns/ http://en.wikibooks.org/wiki/Danish/Nouns