A reader writes:
I propose that “an” was invented to prevent us from having to interrupt the flow of speech. And it still fills that purpose before unaccented first syllables starting with h.
This comment suggests that the indefinite article form an developed from the form a as a means of facilitating pronunciation.
Unlike Esperanto, English is not an invented language, but the product of more than a thousand years of development. An was not invented to facilitate the flow of language. Neither did it begin its life as “an indefinite article.” It started out as a numeric adjective.
The English indefinite article a/an derives from the Old English word for one: ane. The word was written ane, anne, aenne and aene in its various declensions. As it evolved into our modern indefinite article, sometimes it signified the number one and sometimes the article a.
For example, in an OE version of the New Testament parable of the workmen who are all paid the same for different amounts of work, the owner of the vineyard pays them “anne pening,” that is, “one penny.” In the account of the voyages of Ohthere and Wulfstan, Ohthere refers to Skiringssal as “an port,” that is, “a port.”
Even in modern usage, the article a/an can be used in the sense of “one,” as in, “I’ll have a hamburger, a cherry Coke, and two orders of fries.”
Like the reader whose comment prompted this post, some modern speakers feel that that an “still fills [a] purpose before unaccented first syllables starting with h.” According to a note in the OED, “many (perhaps most) writers down to the 19th century retained an before sounded h and some even before eu, u, as “an historian,” “an euphonic vowel,” and “an united appeal.” Most modern usage guides, however, recommend a. That’s not to say that you can’t say “an historic” if you want to.