The Great Vowel Shift

"The Great Vowel Shift is what divides Middle English (the language of Chaucer) from Modern English, the earliest monuments of which include Shakespeare and the King James Bible...

Wikipedia
"The Great Vowel Shift was a major change in the pronunciation of the English language... the two highest long vowels became diphthongs, and the other five underwent an increase in tongue height with one of them coming to the front... The surprising speed and the exact cause of the shift are continuing mysteries..."

The what of today DOES have the initial H sound. Say it, you'll hear it. We write it with the letters reversed from the way we say it: it's pronounced HWAT.

Latin was the language of the Church service and of serious scholarship (that was true throughout western Europe, not just in England). But it was not a language commonly spoken, even among scholars.

French was the language of the court and the nobility, English the language of the common people.

The overwhelming majority of the people spoke English, not French or Latin, but if you wanted to get anywhere with the ruling class, you needed to speak French as well, and if you wanted a career in scholarship or the Church (not just in England but throughout western Europe) you needed to read and write Latin. The typical Norman noble, though, was not literate in Latin (or in French or English, commonly), nor was the typical peasant or townsman.

Over time, the Norman aristocracy ceased thinking of itself as Norman or French and started thinking of itself as English. Anglo-Norman (the Old French dialect of the conquerors) remained the language of the English court until the 15th century, although literature resurfaced in Middle English in the 14th (e.g. Chaucer). By the time of the Tudors, the transformed English language had replaced Norman French as the court tongue.

The Great Vowel shift is indeed the main difference between Middle and Modern English, a difference in pronunciation rather than in writing.
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