Elverhøy lyrics

That's quite a big question, let's see...

Let's start from saying that "modern Norwegian" doesn't actually exist. When you talk about Norwegian, what you mean are two different written forms (bokmål and nynorsk) and a wide variety of dialects spoken in Norway, which differ one from the other at different degrees, some quite significant. The closest overlapping you have between one of these written forms and a spoken dialect occurs with bokmål and the dialect of Norwegian spoken in Oslo, which coincide not perfectly, but significantly.

Dano-Norwegian was an old variety of Norwegian which started to develop when Norway became a part of Denmark and started using the Danish written standard, but pronouncing it as though it were Norwegian. Simply put, they wrote in Danish, but read in Norwegian. Modern days bokmål writing derives from this use of the language. This variety covers a long period of time (almost 3 centuries), so we're talking about not one language, but several varieties that changed significantly from 1500s to 1800s. https://en.wikipedia.org/wiki/Dano-Norwegian

Old Norse is an umbrella term used to indicate several varieties of Nordic languages spoken between the 8th and 15th century (again, a very long period of time, so we're actually talking about several varieties of languages that changed significantly over time). Roughly speaking, Old Norse is divided into two main macro-dialects: Old West Norse and Old East Norse. These two macro categories can be further divided into more varieties. The western variety is the most attested one, and is thus the most studied by sheer amount of material, especially when it comes to Old Icelandic, though Old Norwegian is also widely attested. Modern Norwegian dialects have changed a lot since the times of Old Norse, and none of them, not even the most conservative ones, will grant you easy access to texts that old. Unlike Icelandic, which has remained relatively unchanged since the times of Old Icelandic, especially in the written form, and is therefore a very useful tool to step into the world of Old Norse texts.

Proto-Norse is the common ancestor to all modern Nordic languages. It's the oldest (2nd to 8th century) attested variety of North Germanic language we have, but the corpus of texts (all of which come from short runic inscriptions, mostly from rune stones) is still small and thus a comprehensive study of the whole language is hard to pull. Best we can do is deduce a few common guidelines, but nothing as detailed as Old Icelandic texts allow.