The three periods in the development of the English language are Old English, Middle English and Modern English.
Old English is comprised of many languages, mainly from Germanic invaders that arrived in the northern area England. These invaders, speaking Anglo-Frisian dialects, came from what is now northwest Germany and the Netherlands, and these dialects became the basis of Old English. Approximately one half of the words used in Modern English today are based on Old English words, e.g., "be," "strong" and "water."
With the French invasion by William the Conqueror (of Normandy) in 1066, English through enormous changes. The upper-class Normans spoke French (a Romance language, Latin-based) and the lower-class English did not: but the working classes (that supported the French) spoke English. There was a "linguistic class division" until the 14th Century, when (in a "linguistic shift") English once again became the language of England—however now the language consisted of many French words that we still use today, such as "rendezvous" or "connoisseur." This is what is referred to as Middle English. As with Old English, it would still be very difficult for the modern English-speaking world to understand Middle English.
As a side note, English literature became "respectable" in the 1200s. Edward III was the first monarch to "address Parliament in English." By the end of that century, even the royal court had switched from the Norman French to Middle English. It was during the rule of Edward III that Geoffrey Chaucer, considered by many to be "the father of English literature," was granted a position in the King's household. His varied career assignments, which exposed him to many levels of society, also exposed him to their languages as well.
Chaucer is an especially important figure in the development of the English language in that he developed the "resources of the English language for literary purposes." At that time, English was still thought of as a "rough peasant language," and Latin was the language of the Roman Catholic Church—in its monasteries and "centers of learning" which were dominated by the Church.
By using [late Middle English] instead of the more fashionable French...he added tremendously to its prestige...
Literature to come would be based upon the example of Chaucer's work, and this greatly impacted the use of Middle English rather than French, appealing more to the middle class. Chaucer's writing was popular, and those able to read it had to be able to read Middle English. In other words...
Chaucer is credited by some scholars as the first author to demonstrate the artistic legitimacy of the vernacular Middle English rather than French or Latin.
By the 15th Century, the English language changed yet again. (Because English continues to change, it is known as a "living language.") This change is known as the Great Vowel Shift. This brought English into the third stage, known as Modern English. The results are first seen at that time...
...by the spread of a standardised London-based dialect in government and administration and by the standardising effect of printing.
It is easy to see this when reading Shakespeare: he writes poetically so the sentence structure is unusual and some words are now considered archaic, but we can generally understand the language— which continues to evolve today.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.
The evolution of English, like the evolution of England, tells a interesting if not lengthy and complex story. To define three periods leaves the story of the evolution up for considerable debate. In rough measure, the development can be broken down in 500 year spans; the time from the fall of Rome around 500 up to 1000 can be considered the period of Early or Old English. Middle English existed from about 1000 to 1500; during that time The Great Vowel Shift altered the pronunciation and spelling. The period from the Renaissance to now, 1500 to 2000, can be considered the period of Modern English.
The original Anglo-Saxon dialect from which modern English evolved was already a compendium of different languages in the 400's, absorbing Latin, Germanic, and Celtic influences. By the 800's, the Vikings had introduced Norse into Anglo-Saxon; by 1066, the Normans had introduced Norman French into the language. For centuries, French became the language of diplomacy, Latin the language of the Church, and Greek the language of philosophy and science. All of these contributed over time to the development of English, as did words from Africa, India, North America, and elsewhere where England established her empire.