"Big girls eat vegetables."
Why isn't the "peculiar" french rule used here for de vs. des?
I.e. I wrote... De grandes filles mangent des legumes ... I was following the peculiar rule when there is an adjective before a plural noun. So why is it "des"?
Is it because the adjective is plural and thus the rule no longer applies?
Because the singular of this sentence is "une grande fille..." and in French, "une" has a plural form which is "des".
But you did say that "des" become "de" when it comes before an adjective?
yes, so "de grandes filles".
anyway, the French translation provided by Duo is wrong in my opinion because it is a general statement (universal truth), the kind of sentence you tell your 4 yo daughter to convince her to eat vegetables, so it should be constructed with a definite article to express that generality:
Les grandes filles mangent des légumes.
"les légumes" would translate "the vegetables".
singular in French is "un légume" (one), plural "des légumes" (in English, a/an has no plural form)