"We are eating a salad."
Translation:Nous mangeons une salade.
As roro1996 said, "une salade, des salades" is countable (a/one salad, salads).
If you mean "some salad", you can use "de la salade", remembering that partitive articles are built with definite articles.
"d'une salade", with indefinite article (= of a/one salad), can be used in another construction: j'ai mangé une feuille d'une salade = I ate one salad leaf (one leaf of one salad)
You use de when you have a certain amount of something, J'achete un litre de lait. But you say: J'achete du lait because you don't know the amount of the milk. So I think Nous mangeons de une salade should only be wrong because there is supposed to be dune because of the vowels. Or maybe because it is a certain amount and than you should use de. At least that's what I've been told. :)
You should use a PC which would make you access the Tips&Notes in the lessons.
This is an extract of the Tips&Notes in the Basic1 unit:
French has two grammatical genders: masculine and feminine. All nouns have a gender that you must memorize. Sometimes, the gender can be obvious: une femme ("a woman") is feminine. Other times, it's not obvious: une pomme ("an apple") is also feminine.