Why "Den" instead of "Det" here? I thought "Det" was used when the subject is not defined.
In this case the subject would have been defined because it would have come earlier in the conversation. Based on this sentence, you can presume the "it" they are discussing is a common (-en) word.
If you were translating this sentence from english to danish, and didn't have any other context, den or det would be acceptable.
That's valid English, but it's not something you'd hear in everyday speech -- it's very dramatic.
Which is fine - not every expression in a language is used in everyday speech.
True, but I think it's worth pointing out that if you were to say "It exists no more" to someone, they would most likely give you a strange look.
It's certainly useful to know archaic and unusual expressions, but I think the Danish here is everyday Danish -- please correct me if I'm wrong -- and so it should be translated into everyday English unless you have a good reason not to.
It is unlikely you would hear many English speakers say "That does no longer exist". Please consider changing this to "That no longer exists".
this was suggested to me as the right answer: "It cannot to be found any more." (instead of the correct "It cannot be found anymore." that I wrote. Somebody please correct the mistake!