C1
Article-usage nuances
The article is dropped with generic abstract nouns, generic plurals, professions/nationalities (after sein/werden/als, without an adjective) and material nouns; but it's kept in fixed idioms. When an adjective precedes a profession, the indefinite article returns.
Examples
Er ist Arzt.
He is a doctor. (no article)
Er ist ein guter Arzt.
He is a good doctor. (article returns)
Freiheit ist ein hohes Gut.
Freedom is a high value. (abstract, no article)
Kinder brauchen viel Liebe.
Children need a lot of love. (generic plural)
Der Ring ist aus Gold.
The ring is made of gold. (material)
Er verlor den Kopf.
He lost his head. (fixed idiom keeps the article)