Every language has rules that don’t seem that explainable to a linguist (or anyone else, I presume). One such rule in French is that if you have a plural noun that’s modified by a prenominal adjective (i.e. one of the few French adjectives that comes before the noun), then you use the singular form of the preposition de, not the plural form, which you would otherwise expect:
- journées des collections (no adjective, plural des used)
- Musée des Collections Historiques (postnominal adjective, plural des used)
- de grands collections (prenominal adjective, singular de used)
So, we know that French adjectives generally are postnominal (after the noun)—when do you put them in front of the noun? About.com suggests the acronym BAGS for remembering at least some of the adjectives that go before the noun:
- Good and bad
- Size (except grand for humans)
According to About.com, this phenomenon is related to inherent versus non-inherent properties of the noun that is being modified—a distinction similar to necessary versus accidental qualities, which, according to Pustejovsky’s The Generative Lexicon, is a distinction that goes back to Aristotle. Pustejovsky points out that adjectives describing necessary versus accidental qualities behave differently in the progressive aspect in English, with adjectives describing accidental qualities being grammatical in the progressive, while adjectives describing necessary qualities are not grammatical in the progressive—so that you can say (relevant adjectives in bold):
- The horse is being gentle with her rider.
- You’re being so angry again!
- Stop being so impatient.
…but, according to Pustejovsky, you can’t say:
- * John is being tall today.
- * Aren’t you being beautiful tonight!
- * Stop being so intelligent.
(In linguistics, the * before a sentence means that you can’t say it in the language in question. Note that there’s no claim that you can’t say Aren’t you beautiful tonight—the claim is only about the progressive aspect, indicated in these examples by the verb form being. You are free to argue with Pustejovsky’s claims about whether or not the starred sentences are really ungrammatical. Note also that there are French adjectives whose meaning changes depending on whether they’re prenominal or postnominal—more on those another time.)
So, why the de in front of plural nouns that have prenominal adjectives? I have no idea. The interesting thing to me is not so much the specifics of the rule (use de, not des, in front of a plural prenominal adjective) as what it suggests about the representation that underlies the rule, the qualities that the language has to have in order for a rule to be able to make reference to those qualities: in this case, something like the distinction between inherent versus non-inherent or necessary versus accidental qualities.