Hi Binoy,

The event that started the transition from Old English to Middle English was the Norman Conquest of 1066, when William the Conqueror invaded the island of Britain from his home base in northern France, and settled in his new acquisition along with his nobles and court. William crushed the opposition with a brutal hand and deprived the Anglo-Saxon earls of their property, distributing it to Normans who supported him.

Poet 1 - David Daichess' A Critical Histroy of English Literature.

Poet 2 -  Cambridge Guide to History of Middle English Literature.

Poet 3 - Andrew Sander's book in History of English Literature.

Feel free to ask doubts in the Comment Section.

I hope this information helps you.

Good Luck!