Short answers, pulled from the story.
Old Norse gave rise to the modern North Germanic languages: Icelandic, Faroese, Norwegian, Danish, and Swedish. The extinct Norn language of Orkney and Shetland was also a descendant. Among these, Icelandic, Faroese, and Övdalian have changed the least in grammar over the past thousand years.
In the 11th century, Old Norse was the most widely spoken European language, ranging from Vinland in the west to the Volga River in the east. It was spoken in Scandinavia, Greenland, the Faroes, Ireland, Scotland, northwest England, Normandy, and Kievan Rus'.
Many common English words derive from Old Norse, including egg, knife, sky, window, anger, bag, husband, law, skill, skin, trust, and window. The pronouns they, their, and them also come from Old Norse, replacing the earlier Anglo-Saxon forms híe, hiera, and him.
Old Norse was originally written in the Younger Futhark, a runic alphabet of only 16 letters. After the conversion to Christianity in the 11th century, the Latin alphabet came into use, and the oldest preserved Old Norse texts in the Latin alphabet date from the middle of the 12th century. Runes continued to be used until the 15th century and were recorded in some parts of Sweden as late as the 19th century.
Old West Norse was spoken in Norway and Atlantic settlements including Iceland, Greenland, and the Faroes, while Old East Norse was spoken in Denmark and Sweden. Key differences include the treatment of consonant clusters, with -mp-, -nt-, and -nk- shifting to -pp-, -tt-, and -kk- in Old West Norse around the 7th century, and the monophthongization of diphthongs æi and au, which occurred in Old East Norse starting in mid-10th-century Denmark but not in Old West Norse.
Icelandic orthography was intentionally modelled after Old Norse in the 19th century, keeping spelling largely stable. Contemporary Icelandic speakers can read Old Norse because the written forms differ only slightly in spelling, semantics, and word order, though pronunciation, particularly of vowel phonemes, has changed considerably.