Short answers, pulled from the story.
Middle English began following the Norman Conquest in the year 1066. The ruling class spoke Anglo-Norman French while the common people continued speaking English dialects that evolved from Old English.
Old Norse contact transformed English from a synthetic language with complex inflections into a more analytic language with a stricter word order. Norse immigrants influenced the loss of inflectional endings, which simplified English grammar and made it easier to learn.
The Great Vowel Shift began during the later Middle English period and continued into the Early Modern English era. This process changed the quality of long vowels and diphthongs, creating the disconnect between spelling and pronunciation that characterizes modern English orthography.
Geoffrey Chaucer wrote the most famous Middle English literature in the second half of the 14th century. His work The Canterbury Tales remains the most studied and read work of the period and was written in the emerging London dialect.
By the end of the period around 1470, a standard based on the London dialects had become established. This standard, known as the Chancery Standard, emerged in official documents and formed the core around which Early Modern English developed.
Eth fell out of use during the 13th century and was replaced by thorn, which mostly fell out of use during the 14th century and was replaced by th. Wynn, which represented the phoneme w, was also replaced by w during the 13th century.