With the Norman Invasion in 1066, England was occupied by a French-speaking people, and their language affected the speech of the natives. This influence, coupled with the natural evolution of the language, brought about some significant changes, and we can say that by about 1100, English had entered a new phase.
Whereas OE requires extensive study before a novice can begin to read it, some ME requires only a few glosses on difficult words and constructions, once you've learned to get past the spelling. Here, for instance, is a passage from one of Chaucer's most famous fabliaux, The Miller's Tale:
Derk was the night as pich or as the cole,It requires just a little practice and a few notes to read this: "Dark was the night as pitch or as the coal, And at the window out she put her hole, And Absolon, it befell him no better nor worse, But with his mouth he kissed her naked ass [arse], With full savor, before he was aware of this." (The bawdiness is perfectly in character for Chaucer, mind you.)
And at the windowe out she putte hir hole,
And Absolon, him fil no bet ne wers,
But with his mouth he kiste hir naked ers,
Ful savourly, er he were war of this.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.