Many words in the English language have changed their spellings over the centuries. Those which date back to Middle English in Mediaeval times have changed more than most; those which are even older are barely recognisable from their Old English equivalents.

For all of that, the pronunciation of words has probably not varied as much as the spellings, and if we were to find ourselves transported back to an earlier era, we may find that we would be able to understand a lot of what was being spoken, even if it seemed unrecognisable when written down.

There are a few words which have changed their spellings as a result of them being used with an indefinite article in front. One of the best-known examples is the species of snake known in English as an adder .

Adder was originally spelled naedre in Old English. It was used as a generic name for a serpent, as well as a term for the devil. The word survived into Middle English in the 12th-14th Centuries, by which time it was spelled 'naddre'. When you spoke about one of these snakes, you would call it 'a naddre',