It's just that type of past tense. Look it up, but in English also (and i suspect in many languages), you can get things like that: "You have now reached level 13" or like the sentence we have here, you could easily translate it as: "You have now learned it"
@Babykage: i don't know what you call "expression" here, but i think it is a pretty usable sentence. Not sounding weird at all, to my ear at least
Very interesting to see how expressions have nothing to do with translation now. In other words, learning idioms or expressions or just punching keys here (on DL) like a monkey will actually do the trick of learning the expression.
DL actually teaches you that translations require 1/2 getting accustomed to what natives express.
In my opinion, the literal translation would be: "Now you have experienced it". So, Duolingos suggested translation "Now you know" is a shortened version and more straight to the point.
However, I believe that Duolingo could have communicated it more clear by presenting "Now you have experienced it" as their primary translation. It still is a sentence that sounds like reasonable english in my ears.
Erfahren means something along the lines of learning information through hearing or experiencing something. That is why the word "erfahren" can be translated as "to hear", "to experience" and "to know".
"Ich habe erfahren, dass du deinen Job verloren hast" (I heard that you lost your job)
"Jetzt hast du es erfahren" (Now you know)
It is distinguished from "lernen" which is more about the acquisition of skill or knowledge.
As ozrenllic said, verbs with inseparable prefixes won't take that additional ge- prefixe (check http://en.wikipedia.org/wiki/German_verbs#Inseparable_prefixes), but there are also other ones, like borrowed verbs (with infinitives often ending in -ieren). On the same page, actually: http://en.wikipedia.org/wiki/German_verbs#Past_participle
Note the case of separable prefixes: the "ge" sneaks in between! anrufen -> angerufen (i hope the example is alright, but anyhow you get the gist)
what if the verb in question is "machen"? In English, you might hear someone say, "NOW you've done it!" or "Now you did it!" (Perhaps the comment might be a rebuke to someone whose recent action is to blame for a newly-encountered problem, like your companion just triggered a ❤❤❤❤❤-trap.) So might that be "Jetzt hast du es gemacht!"?
Ah, when Duo gets clever...
Just a moment ago, my previous practice question was to match "stated" to one of gestellt, knapp, or (something else I'm forgetting). Well it was clearly not knapp or the other. So I chose gestellt. Turns out, I was correct. "Well, I didn't know that," I said to my phone with a hint of hostility. I clicked Next. The very next practice question was this one.