when do uncountable nouns take articles before them?

Answer 1

when they are the subject of a sentence: the people are right; the rookies are good; the fish are yummy;

not when they are complements: I love people; I wich I could eat eat fish; who knows when rookies will succeed.

that may not cover all cases, but I think it's a pretty accurate rule of thumb

Answer 2

For the love of Pete, who knows the rule? Could it be when followed by a preposition?