One recent vowel phoneme in English is /ɜ:/. It would seem that this sound only developed in a certain phonetic environment, or to phrase it differently: it only appeared under certain conditions.
There is Old English, and there is the English we speak now. When did exactly did the British (or Americans) change from speaking Old English to speaking the current form of English?
Apparently the word "nightmare" has only been used in the sense of "bad dream" since c. 1829. Before then the term referred to the agent causing the dreams—a mare < mera, mære 'goblin, ...
I'm interested in learning Archaic English. As a starting point, I guess simple texts that are easy to comprehend would be a good choice. I would appreciate any suggestions.