Short answers, pulled from the story.
Middle English refers to the forms of the English language spoken in England after the Norman Conquest of 1066 until the late 15th century, roughly coinciding with the High and Late Middle Ages. It was succeeded in England by Early Modern English, which lasted until about 1650.
The Norman Conquest replaced England's political and ecclesiastical leadership with Norman rulers who spoke Anglo-Norman French, making French the prestige language of literature, law, and administration. This caused Old English to lose its written standard and introduced a large body of Anglo-Norman vocabulary into English, including legal terms such as court, judge, jury, and parliament, alongside paired synonyms like pig and pork, and freedom and liberty.
Old Norse contact in the Danelaw, which covered approximately Yorkshire, the central and eastern Midlands, and the East of England, led to the erosion of inflectional endings in both Old Norse and Old English, as mixed communities simplified grammar to communicate. Old Norse contributed the third-person plural pronouns and influenced modals, comparatives, and common adverbs such as hence and together.
Geoffrey Chaucer, writing in the second half of the 14th century, is the best known writer of Middle English; his Canterbury Tales remains the most studied work of the period. John Wycliffe was also prominent during the 14th century, producing early translations of the Bible into English.
The Chancery Standard was an official written English dialect that emerged around 1430, rooted in the East Midlands-influenced speech of London. It originated in August 1417 when King Henry V began issuing Signet letters in English, and it largely formed the basis for Modern English spelling, even though pronunciation has changed considerably since.
Middle English spelling was generally consistent with pronunciation at the time, but the Great Vowel Shift, which began in the later Middle English period, changed how long vowels were spoken while spelling conventions remained fixed. The Chancery Standard locked in many spellings before the shift was complete, creating the persistent mismatch between letters and sounds in Modern English.