The takes either a singular or a plural subject. A/an only takes the singular.
When we pluralize a noun preceded by an indefinite article, we simply drop the article (sometimes replacing it with some). Why is this?
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
whereas, "a happy man" is:
As one can see, only the adjective changes.
As for the Old English indefinite article, my conjecture is that the process never went through for a number of possible reasons:
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...
As pointed out elsewhere, language is the result of an evolutionary process, not logical design.
The origins of the word "the" aren't connected with those of "a/an", so there's no reason why they should share all characteristics.
As OP says, "some" can function as a kind of 'plural' for "a/an". So can "a few", "a number of", etc. In some contexts, "any" can be used as the pluralised version of "a/an". I'm not overly concerned about the scope of the term "indefinite article" – it's just a (sometimes enlightening) name we often use, not a 'pre-existing' class into which any given word either falls or doesn't.
It's not as if our language is seriously restricted by not allowing for "a/an" to be used of multiple subjects. And after all, in some contexts "a" can effectively refer to multiple subjects where "the" implies a single one...
LATER: More specifically addressing OP's question as to why "a/an" can't be pluralised the same as "the". Firstly, note that in the above example, "a" is effectively pluralised – as becomes clear when you realise it means "presidents in general" rather than "a randomly selected president".
Secondly, consider "Recipe: Mix some cloves, a cinnamon stick, and apples in a bowl". There's no need for an article when we pluralise "an apple" there.
Thirdly, as @Robusto implies, some/many/most/all contexts where you would use the indefinite article in reference to "one of it" simply don't lead to meaningful contexts if there is more than one of it.
Fourthly, "a" can mean exactly "one", particularly in contexts associated with 'countability' (so can "the", but more in the context of 'identification'). This makes us leery of using it around plural subjects, because we sense it sits uneasily with 'one-ness' of "a".
When you are referring to a specific item or specific items, you use "the", like
When you are referring to a "nonspecific" item, you use "a" for the singular, like
If you were to leave out "a", you would get
How many steaks do you have? One massive steak? Two steaks? Exactly π steaks? 4/5 of a steak? Without the article, "steak" in this example becomes a "non-counted" entity; you aren't indicating anything about whether there are discrete items. In
you are referring to a collection of discrete items. When you say
though, it is clear that you are referring to discrete items. If "steaks" is plural, you have to be able to count them and therefore have to have more than one discrete item. The article isn't necessary. However, you can say
According to the Wikipedia entry for "article (grammar)", "The articles in the English language are the and a/an, and (in some contexts) some." (emphasis added) In the case of talking about "indefinite objects", the article isn't needed to show that you are talking about discrete items or specific items.
If a/an was derived from an, which was derived from the number one, there logically wouldn't be any corresponding derivation of a plural indefinite article. So modern English doesn't have one.
Because in English the indefinite articles are an and a; some, a, an, the, and other words are classified as determiners.
The NOAD that comes with the Mac OS X 10.7 ("New Oxford American Dictionary 3rd edition © 2010 by Oxford University Press, Inc.") defines the articles as determiners.
Probably because you don't need an article with plural nouns.
You would use definite plural pronouns if you want to refer to a specific group within a larger set.
The plural indefinite is probably not in use because it would be superfluous given other more efficent ways of expressing the notion of indefinite.
In English, we do not inflect adjectives by number. In particular, we do not inflect articles by number. (Unlike French ... l'haricot vert and les haricots verts.)