In my opinion knowledge of the history of English is a must for an English teacher. You need to know at least the basic facts - what tribes invaded Britain and what impact this had on the development of the language.
I've always found this topic fascinating so I'll try to explain the basics in my own words.
You all know (I mean all my students, hopefully) that in 5 century A.D. Britain was invaded by the tribes from Germany - the Angles, Saxons and Jutes. It's curious that Old English spoken by them is much closer to German than to Modern English, even English native speakers can't understand it. Here's a short example:
It sounds like an incantation, doesn't it?
1066 was an important turning point in the history of the language as this is the year when William the Conqueror, the Duke of Normandy, defeated the Anglo-Saxons in the Battle of Hastings and the Norman Conquest began. French became the standard language of courts, parliament and society. The Norman dialects of the ruling classes mixed with the Anglo-Saxon of the people and Anglo-Saxon gradually transformed into Middle English. Roughly speaking Middle English (used in 12 - 15 centuries) was created as the mixture of French spoken by the Normans and Old English spoken by the Anglo-Saxons.
Middle English is challenging to read. The early ballads about Robin Hood were written in this form of English. Here you can have a glimpse of a text in Middle English - A Gest of Robin Hood (and maybe even try to plough through it).
In the late 14 century began the Great Vowel Shift, a massive sound change affecting the long vowels of English during 15 to 18 centuries. It was completed in about 1550 and resulted in Modern English. So Shakespeare's works were written in this form of the language which is good news - you'll be able to read his plays in the original with a little bit of effort.
Of course, modern English vocabulary is not just about French borrowings. This video will tell you about the words borrowed from different languages.
