First of all, you need to distinguish between indefinite and definite articles. The first category indicates one or more out of a class of items which have the qualities defining this class but aren't specified otherwise:
Wir haben Kinder, unsere Nachbarn nicht.
Ich esse jeden Tag einen Apfel.
Of course you can assign additional qualities to them in order to make them more specific:
Wir haben zwei großgewachsene Kinder, Christine und Markus.
Da draußen steht ein rotes Auto.
The plural of nouns with indefinite articles goes without article:
Ich esse jeden Tag Äpfel.
Da draußen stehen rote Autos.
The second category always implies one or more special items out of a class of items which you can clearly tell from the other items by certain properties. As shown above, those properties must have been assigned earlier, otherwise we don't know what the term is referring to:
Ich esse jeden Tag die Äpfel. (What apples??) but:
Die roten Autos (those we've talked about earlier) stehen jetzt schon drei Tage lang da draußen, weißt du, wem die gehören?
In addition to this rule, there are terms describing what doesn't exist in distinct forms but as a kind of continuous flow, e.g.
Raum, Zeit, Luft, Erde, Feuer, Wasser etc.
Same with food like Milch, Butter, Mineralwasser, Fisch, Fleisch, Gemüse, Obst, etc., as generalized terms also for kinds of food which exist in natural units like fish, apples, and oranges. Thus you say:
- Gemüse ist ein wichtiger Bestandteil unserer Nahrung.
(a general term
without need to get specific)
- Heute koche ich Gemüse
(indicating a certain kind of food which will be part of today's menu, but the vegetables themselves might not yet been purchased or even been decided upon)
- Ich wasche Gemüse, bevor ich es verwende
(a more general context with no need of vegetables being present in order to refer to them)
- Ich wasche das Gemüse, bevor ich es verwende
(a specific context, e.g. in a cooking seminar where the instructor shows every step in practice, and we know what vegetables they are speaking about. Also, as the vegetables are already taken for granted, so to say, the emphasis here lies on "ich wasche")
- Gibst du mir bitte das Gemüse?
(please hand me the cooked vegetables which are on a plate on the table)
Reviewing those examples I think vegetables are not ideal because they also can be handled piece by piece. So, let's add some wine-and-water examples:
Ich trinke gerne Wein.
Der Wein ist heute besonders gut. (The wine served on the table today)
Zum Wein wird auch gerne Wasser getrunken.
Wird dem Wasser Flour zugesetzt? (the drinking water coming from the faucets in the house)
and so on.
And, of course, the same goes with feelings as well as other abstract concepts in our lives, like time. Usually, you don't specify the amount of liking, delight, passion, discomfort, anxiety, etc. beyond saying what the object of that feeling is, or expressing its "size". In these cases, you don't use an article, indefinite or not:
Ich habe Lust, heute ins Kino zu gehen.
Schade, dafür habe ich leider keine Zeit.
Ich habe Angst, so spät noch heimzufahren.
Ich hatte heute viel Stress im Büro.
Die Kinder haben mir gestern mit ihrem Geschenk sehr viel Freude gemacht.
Ich hatte solche Lust, alles hinzuschmeißen.
Abstract nouns with definitive articles must be assigned a specific quality which makes them distiguishable from its general meaning:
Die Angst zu versagen ist einer der Hauptgründe für Stress und Krankheiten im Berufsalltag.
Die Freude am Leben ist oft unabhängig vom materiellen Status eines Menschen.
In der Zeit, die du benötigst, um einen Apfel zu schälen, habe ich schon ein ganzes Menü gekocht.
