The excellent /u/bloodswan answered a question of mine on a similar topic with great detail: When I hear Chaucer spoken in Middle English, it almost sounds more like Dutch than English. But though Shakespeare is closer in time to Chaucer than the present, his English is much easier to understand. What happened to English in between, and why? Hopefully that should give you a good background to the slow evolution of English as a language.
See also this answer from /u/cdesmoulins, as well as the linked posts