The Germanic group of Indo-European languages split into three sub-groups – East, West and North Germanic. East Germanic, now extinct, included Gothic, in which the earliest example of a Germanic language in writing – the extracts from the Bible translated by Ulfilas – still exists. This manuscript dates to the 4th century, and is housed at Uppsala in Sweden (having been looted from Prague during the Thirty Years War).
West Germanic gave rise to present-day German, Dutch, English, Frisian and Afrikaans.
North Germanic, which comprises the Scandinavian languages, itself divided into two: West and East. West Scandinavian is now represented by Icelandic and Norwegian (and Faroese) while East Scandinavian is Danish and Swedish. These are the historical divisions, but there are various features of the languages which cross these boundaries in different ways.
The most obvious is that Swedish and Norwegian share ‘tonelag’ – a feature unusual in European languages, of tonal distinctions like those in Chinese. Pairs of two-syllable words may be distinguished solely by whether they are spoken on Tone 1 or Tone 2. However, in dialects of Northern Norway, this feature has disappeared, leaving pairs such as “bønder’ (“farmers”) and ‘bønner’ (“beans”) homophonous. I don’t know if the same has happened in the Swedish spoken in the north of that country – perhaps someone can enlighten me? To complicate matters still further, the tones may vary from one dialect to another, just like vowels and consonants.
Neither Danish nor Icelandic have ‘tonelag’. Danish, however, does have a reflex of it, called ‘stød’, which is a glottal stop inserted in one member of the pair. Just as the spelling of Norwegian and Swedish gives no indication of the tone pattern on their bisyllables, so Danish does not reliably indicate in its spelling the presence or absence of ‘stød’ – and an added complication is that sometimes the glottal stop, which comes in the coda of the syllable, precedes a final consonant, and at other times follows it. The dialects of the extreme south of Jutland have lost the stød – which puts them in a similar situation to the dialects of northern Norway. This is fortunate for foreign learners, who can just ignore both tonelag and stød without causing too much confusion in their listeners!
On top of this, Norwegian Bokmål often looks like Danish, while Nynorsk can resemble Swedish. In Danish, the word for “week”, for instance, is ‘uge’. In Bokmål it is ‘uke’. In Nynorsk it is ‘veke’, and in Swedish ‘vecka’. In Icelandic, ‘vika’. Here Bokmål and Danish go together. Icelandic – which ‘should’ be with Norwegian, agrees with Nynorsk, but both are close to Swedish.
The isoglosses are difficult to sort out across the Norse area. Icelandic has retained a lot of features of Old Norse, and is more different from the other three than these are amongst themselves. The speakers of Danish, Swedish and Norwegian get along very well without the need for translators, and it always surprises me that fiction for adults written in one of the three may be translated into the other two.