The best way to teach the history of the English language is through a literary survey course, focusing on morphology, linguistics, and transformational grammar.
English is a Germanic language, a collection of several dialects that merged in the Anglo-Saxon period following the fall of the Roman Empire. This language is generally called Old English. A good example of this version of early English is found in the epic poem Beowulf:
Hwæt! Wē Gār-Dena in geārdagum, hū ðā æþelingas
þēodcyninga, þrym gefrūnon, ellen fremedon.
During the Middle Ages, the Catholic Church greatly influenced language, as monks were the book-keepers. English evolved to look more similar than the language of today. Chaucer's Canterbury Tales is a good example of Middle English:
Whan that Aprille with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;
The invention of paper and the printing press in the 15th Century gave way to the English of the Renaissance. Shakespeare's plays were good examples of early Modern English:
Since then, late modern English has become more uniform in spelling and pronunciation thanks to the dictionary, education, and the proliferation of books.
Along with other influences upon English, the Danes affected it early on. In fact, Old English began with the Angles, a people who originated in a section of Denmark still called Angeln. First, they contributed at least 900 words, such as husbondi and syster (husband and sister). Second, the Danes added pronouns to English, filling a real linguistic need. Most significantly, before the arrival of the Danes, Old Englsh conveyed meaning through word endings, like Latin, instead of word order. The Danes simplified language by elminating this use of inflection. This process was completed when Middle English evolved.
With the Norman Invasion of 1066, the language of the nobility became French after William the Conqueror became king. He confiscated the lands of the Anglo Saxon nobility, allowing the Normans to control government at all levels. Consequently French became the official language. Medieval literature was in French, such as the tales of King Arthur. To this day over half of the words in English are of French derivation.
With reference to Geoffrey Chaucer, he was in touch with French poets and translated poetry and wrote his own in French and sought to reproduce both the matter and style of Fench poetry in his own. But, he decided to use the English of his day for his Canterbury Tales.