You are right, of course. I didn't want to delve into that sort of complexity, because I didn't want to confuse the issue terribly much. Nonetheless, the main point regarding Icelandic that I wanted to make was about the mutual intelligibility with Old Norse, and the fact that, as languages go, it is extremely conservative (meaning that it resisted change for a long time).

For instance:

> As a testimonial from those times, the author of the thirteenth century Icelandic Gunnlaugs saga ormstungu makes a reference to the spoken English language in the time of Anglo-Saxon King Ethelred (986 – 1016 AD):

> Ein var þá tungu á Englandi sem í Nóregi ok í Danmörku.
“One was the tongue in England as in Norway and in Denmark” [0]

Interestingly, this sentence renders identically into Norwegian (Bokmål) as into Danish: Den ene var tungen i England som i Norge og i Danmark. [1, 2]

In Swedish, in case you were wondering, it translates as En var tungan i England som i Norge och i Danmark. [3]

In Icelandic: Ein var tungan í Englandi eins og í Noregi og í Danmörku. Notice how similar these are to the words of a man who's been dead for a millenium.

Google Translate doesn't have nynorsk or Faroese available, but, I expect these would render into something between Swedish and Bokmål; and, something nearly identical to Icelandic; respectively.

I also didn't mention modern English's closest living relative, West Frisian. [4, 5]. Old Frisian [6] and Old English were also mutually intelligible, even moreso than Old English and Old Norse. There is still a subset of modern West Frisian that, although written much differently from its English translation, sounds nearly identical. [7] My experience of it was that it was as if one pronounced Dutch with English vowels.

So, that's a fuller exposition of the family tree of Old English. Things start getting weirder in the Middle English period, when the Normans conquer England in 1066, causing English to start accreting French loan words while, at the same time, dropping the case system and most of its inflections, and acquiring silent letters. [8]

If you want to put this all together into a fuller story of how the sounds of Anglo Saxon were reconstructed, you have to know about vowel shifts [9], and, maybe a bit about consonant shifts [10].

Once you have those tools at hand, plus the rest of a graduate course in comparative linguistics, you can essentially reverse engineer the vowel and consonant shifts to reconstruct a probable phonology for Old English.

But, that would have made a much longer comment, you see. ;)

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[0]: https://wordwidefx.com/en/blog/post/old-norse-and-old-englis...

[1]: I used Google Translate for this, but, I did study Norwegian for a bit, and, although my norsk is a bit rusty, this looks correct to me.

[2]: This is not a coincidence. Norway was essentially under Danish rule from 1523-1814. See https://en.wikipedia.org/wiki/History_of_Norway

[3]: Google Translate again. But, based on my limited knowledge of Norwegian and Swedish, this, again looks right.

[4]: https://en.wikipedia.org/wiki/West_Frisian_language

[5]: https://en.wikipedia.org/wiki/Anglo-Frisian_languages

[6]: https://en.wikipedia.org/wiki/Old_Frisian

[7]: https://english.stackexchange.com/questions/113646/why-is-it...

[8]: For instance, in Middle English, the word "knight" was spelled the same as it is today, but pronounced /kniçt/. In Old English, it was spelled variously as cniht, cneoht, cnyht, cneht, cnieht, and maybe even 1 or 2 more variants, but still pronounced the same as in Middle English.

English spelling didn't start to standardize until William Caxton introduced the printing press to England in the late 15th century.

[9]: https://en.wikipedia.org/wiki/Vowel_shift

[10]: https://encyclopedia2.thefreedictionary.com/Consonant+Shift