well many historical factors tbh. The viking invasions for one. Some Norsemen who invaded stayed and of course when an adult learns a language they wont learn it perfectly. They will mispronounce (well at the time it was considered a mispronounciation), they will simplify and that will inevitably influence the course of a language's development. In addition the loss at the battle of hastings which led to the normans ruling england. Here we get the enormous influx of french and latinate words, so much so that many native english words were displaced in favor of loanwords like question, danger and so on. This continued for centuries, and had renewed intensification when france and french culture was aped across europe in the 17th and 18th centuries. Plus english never had a strong purist movement unlike german or icelandic
Some scholars believe the shift was influenced by the rapid migration of people from northern England to the southeast part of the country to escape the Black Death that killed over 25 million people across Europe. The migration resulted in the mixing of accents, warranting changes in the standard London dialect. Others argue the Great Vowel Shift occurred in response to an increase in the number of French loanwords used in the English language. Additionally, some claim the shift was caused by a hypercorrection in response to the increase in the number of French loanwords, which occurred either intentionally or unintentionally, to make English vowels sound less like French
well many historical factors tbh. The viking invasions for one. Some Norsemen who invaded stayed and of course when an adult learns a language they wont learn it perfectly. They will mispronounce (well at the time it was considered a mispronounciation), they will simplify and that will inevitably influence the course of a language's development. In addition the loss at the battle of hastings which led to the normans ruling england. Here we get the enormous influx of french and latinate words, so much so that many native english words were displaced in favor of loanwords like question, danger and so on. This continued for centuries, and had renewed intensification when france and french culture was aped across europe in the 17th and 18th centuries. Plus english never had a strong purist movement unlike german or icelandic
From a description on the great vowel shift:
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