grammar - Why is there no plural indefinite article?

In most languages indefinite articles stem from that language's word for one. For instance in French un, or in German ein, In Italian and Spanish uno or in Portuguese um.

English is no exception: an was derived from one. Note that an was the original indefinite article; the shorter a came later when the final "n" was dropped before consonants.

In some of the languages I mentioned above, the plural form of the indefinite articles is simply formed by applying the noun plural inflection: unos/unas or uns/umas.

In others, such as German and Italian, there is no plural form to the indefinite article. Italian use the partitive article degli/delle as a substitute and this is probably also the origin of the French plural form des.

For some reason English did not go through this last step either. To understand why we need to go back to the way Old English solved the problem.

In Old English adjectives have a different declension depending on whether the noun they qualify is determined or not.

"The glad man" reads

se glæd guma

whereas, "a happy man" is:

glæda guma

As one can see, only the adjective changes.
For one given adjective, you could therefore have different inflections depending on:
- the noun gender (masculine, feminine, neuter)
- the noun being singular or plural
- the four cases (nominative, accusative, genitive, dative)
- whether the reference is definite or indefinite.
So that the same adjective would have to follow either the "definite" declension or one of three "indefinite" declensions.

þa glædan guman

Edit
<conjecture>
The theory I'm trying to check (community please feel free to edit) is that in various languages (Icelandic for a language very close to Old English or Romanian) the article is added as a suffix to the noun. Then it often "detaches" and passes in front of the noun. Icelandic is half way through for the definite article in that matter.

As for the Old English indefinite article, my conjecture is that the process never went through for a number of possible reasons:
- The "loss of inflection" of early Middle English won the race
- The plural of "an" was not easy to evolve at that time (the Romance "-s" plural had not imposed itself yet).
</conjecture>

But the need is still there, just as in any other language where a specific word emerged for the plural indefinite article. This gap is filled by placeholders such as some or a number of.

Most linguist agree that Proto Indo European did not use articles.
Latin does not have any kind of article, and Ancient Greek arguably had no indefinite article either - it was using something very much like present-day English some (τις - "a certain"). And I believe that Old German did not have any article either.

It is a very remarkable fact that articles appeared in many modern Indo European languages in a largely mutually independent yet very similar manner. My feeling is that their emergence compensates for the gradual loss of inflection in these languages. But then present-day German is a powerful counterexample...