What is it called when English speakers, over a long period of time, start adding the letter "n" to the beginning of a word by accident, due to use of the article "an"? For instance, I read somewhere that the word "newt" came from people mistaking "an ewt" for "a newt". I remember there is a term for it, but it's not coming to me.
It’s called metanalysis or rebracketing, amongst many other things:
There are many famous examples of this in English. The rebracketing can go either way, making longer words:
Or shorter ones:
Even orange may be one such, although if so, this happened before it reached English. Note the last part in the OED etymology for orange in English and others:
They aren’t quite sure of that one, though. But it does seem likely.
This process is of course still going on every day, and it may eventually lead to new words, or at least new spellings. Think of the catch-phrase “Texas: it’s a whole nother country”, where another is rebracketed as a nother thereby allowing the insertion of whole between the article and what follows.
This can occur at the phrase level, too. Linguist John McWhorter has an amusing example of “Gladly, the cross-eyed bear” being heard in the hymn line “Gladly the cross I’d bear”. That’s essentially the same process that leads to hilarious (or confusing) eggcorns appearing in print, like “for all intensive purposes” or “when all is set and done”.