Short answers, pulled from the story.
Middle English began following the Norman invasion in 1066 when Old French replaced English at the top levels of political and ecclesiastical hierarchies. The general population continued to speak the same dialects as before the Conquest, but written records shifted toward Anglo-Norman.
The main dialects of Middle English included Northern, East Midland, West Midland, and Southern varieties within England. Additional regional forms included Early Scots, Irish Fingallian, and Yola.
Contact with Old Norse caused the erosion of inflections in both languages, transforming English from a synthetic language with free word order into an analytic language with stricter word order. This grammatical simplification occurred most strongly in dialects under Danish control covering Yorkshire, the central and eastern Midlands, and the East of England during the 10th and 11th centuries.
John Wycliffe and Geoffrey Chaucer produced significant literature during the 14th century, with Chaucer writing his Canterbury Tales in the second half of that century. Chaucer used the emerging London dialect while also portraying characters speaking northern dialects in works like The Reeve's Tale.
A standard based on London dialects known as Chancery Standard had become established by the end of the period around 1470. This standard emerged in official documents and formed the basis for Modern English spelling after being stabilized by Richard Pynson and William Caxton's printing press developed during the 1470s.