When do I not use an article at all (zero article)?

The zero article is used in several situations:

Uncountable nouns/materials: Ich trinke Wasser. (I drink water.)

Professions/Nationalities (without adjective): Er ist Arzt. (He is a doctor.)

Plural nouns in general statements: Ich mag Äpfel. (I like apples.)

Names of cities/countries (without adjective): Ich fahre nach Berlin. (I'm driving to Berlin.)

Abstract nouns: Sie hat Mut. (She has courage.)

Fixed expressions: Auto fahren, Klavier spielen, Hunger haben.