In a Nutshell : it would be somewhere between 400 to 500 years ago => approximately from the mid-1500’s, ie the mid 16th Century.
In order to justify this let’s compare how the speech of ‘English’ speakers sounded in Chaucer’s time, the late 14th Century, with that in the late 16th Century – at the time of Shakespeare.
Here’s the first four lines from the Prologue to Chaucer’s Cantebury tales, in the “original” “language” (which is still very much English, just an “older” version of it) :
Whan that aprill with his shoures soote
The droghte of march hath perced to the roote,
And bathed every veyne in swich licour
Of which vertu engendred is the flour;
And a more up-to-date translation :
When April with his showers sweet with fruit
The drought of March has pierced unto the root
And bathed each vein with liquor that has power
To generate therein and sire the flower;
Once you see the original written version against the modern version then its clear that most untrained English speakers would probably be able to work it out and apply it to other written works. However, modern English speakers when faced with someone actually speaking this would struggle much more – this is how it would have sounded ….
Let’s compare this with what Shakespeare’s English would have sounded like in the late 16th Century ….
So What Happend?
The reason why modern day English speakers would struggle so much with late 14th & 15th Century English is the sound of long vowels.
Between the 15th and the 16th centuries something occurred to the English language that linguistic historians call “the great vowel shift” which affected the sound of English’s long vowels. For example, a word like “goose” would have been pronounced “goas” (oa as in boat) in the 15th century and it would not be pronounced “goos” until sometime in the 16th century. “mice” would have been pronounced “mees”, “house” –“hoos”; “name” — “nahm”, “feet” — “fayt” (ay as in “pay”), etc. The language was in a radical shift during the 15th century and into at least the middle of the 16th century.
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