"Il y a des tomates rouges."
Translation:There are red tomatoes.
How are you supposed to be able to tell the difference between singular and plural by audio alone?
Where indefinite articles are concerned, singular nouns are preceded by the article "de" (sounds like "duh") whereas plural nouns are preceded by the article "des" (sounds like "day"). The hint is in what's spoken before the noun.
Note that "de" and "des" are not exactly pronounced "duh" and "day" respectively; these comparisons are meant to illustrate only the very basic difference in their pronunciation towards distinguishing them. To properly appreciate and mimic French phonology, I definitely recommend looking up a phonics guide with audio samples. (YouTube's a great place to start.)
"Des" is used for indefinite unmodified masculine and feminine plural nouns, unless the indefinite nouns are modified, in which case "de" is used ("Des livres" and "Des femmes" but "De grandes femmes" and "De livres rouges"). "Du" is used for singular masculine indefinite uncountable nouns ("du riz", and "du pain"). "De la" is used for singular feminine indefinite uncountable nouns ("de la poule") unless the noun starts with a vowel or unaspirated "h", in which case it becomes "de l' " ("de l'eau" or "de l'huile")
"Des" is only changed to "de" in front of an adjective:
de + adj + noun
In this sentence the adjective follows the noun so it looks like this:
des + noun + adj
t'as raison! but in french the BANGS rule applies when the adjective goes before the noun.
B- beauty (belle, beau, joli, etc)
A- age (jeune, vieux, vielle)
N- number (dix-huit, vingt, cent, etc)
G- goodness (bien, mauvais, etc)
S- size (petit, grand, gros, enorme, etc)