Is Duolingo correct about de/des?
Duolingo asks to give the correct translation(s) for: "You are young boys." I selected "Vous êtes des jeunes garçons," but this was marked wrong because I missed "Vous êtes de jeunes garçons." I think this is wrong. Shouldn't the preposition always correspond with the singular or plural noun? What would also have been correct is to omit the proposition altogether: "Vous êtes jeunes garçons."
The partitive article (du, de la or des) is replaced by de (or d') in some cases, including in the case where an adjective is followed by a plural noun. For example, de grands enfants = (some) tall children, de petites villes = (some) small towns. Note, though, that if the adjective comes after the noun, des does not change. For example, des résultats encourageants = encouraging results. Most French adjectives usually follow the noun, of course, but there's a handful that come before, including beau, bon, court, gentil, grand, gros, haut, jeune, joli, long, mauvais, méchant, meilleur, moindre, petit, pire, vieux and vilain.
What simonmcg has explained is prefectly right, although there are a good bunch of exceptions to those rules:
1.(too) many native French people simply ignore the rule 'de jeunes garçons' and say 'des jeunes garçons'.
all the adjectives listed can also be placed after the noun in oral French, most often to stress the adjective itself : 'un garçon jeune et beau' is as correct as 'un jeune et beau garçon'.
it is right that 'des' does not change to 'de' when the adjective is after the noun: 'des garçons honnêtes'. The reason is that this is just the plural of 'un garçon honnête', where un/des is an indefinite article, NOT a partitive article.
Merci à tous. I found a great exercise for this particular issue online: http://www.lepointdufle.net/ressources_fle/articles_de_du1.htm. Check it out.
@elka: you're right, it is tough to learn French and as I already said a couple of times in this section, this is the work of a life time... now I wish simple questions would mean easy answers, but this is not always possible, particularly if you consider that we are not professional teachers, just learners like you and we cannot adapt our answers to your potential of understanding because we don't know where you're coming from (your native language?) to give meaningful examples or compare with what may be already familiar to you.
I'm sure you're right that there are some very subtle and complex possibilities in answer to many questions. However I think it's worth thanking people who address just the issue one was wondering abou in a manageable form, as well as the more complicated answers for other people. After all, if one is only learning or polishing up by Duolingo one can't be very advanced! So thanks to you to, but Remy is a great resource.