I found confusing when we use indefinite article in English but no article in Norwegian. This post summarizes the main rules for when to use no article in Norwegian.

Nationality, profession, etc

With nouns denoting nationality, profession, trade, political or religious belief:

Norsk English
Han er lærer. He is a teacher.
Wolfgang er tysker. Wolfgang is a German.
Sofia er kommunist. Sofia is a communist.
Han er kristen. He is a Christian.

Exceptions: We need to use an article when

  1. the noun is modified by an adjective:

    Han er en dyktig lærer.
    → He is a skilled teacher.

  2. the noun is qualified by a restrictive relative clause:

    Han er en lærer som kan sine ting.
    → He is a teacher who knows his stuff.

  3. the noun is of a more subjective nature

    Mor er et geni / en engel / en kverulant.
    → Mother is a genius / an angel / a troublemaker.

After «som»

“As a …”, no article after «som».

Som professor fikk hun det meget travelt.
→ As a professor, she got very busy.

Som barn var han smart og kreativ.
→ As a child, he was smart and creative.

Generality

With singular count nouns when a general idea is inferred rather than a specific example. The noun is unmodified:

Det er vanskelig å skaffe leilighet i Oslo.
→ It is difficult to get an apartment in Oslo.

Vi går ikke ut på restaurant så ofte.
→ We don’t go out to a restaurant so often.

Anna venter barn.
→ Anna is expecting a baby.

Håkon skriver brev.
→ Håkon is writing a letter (or several letters).

Familien har hund.
→ The family has a dog.

After «ha» or «få»

After the verb «ha» and «få» and with an adjective attribute the naked noun is often found in phrases describing the characteristic external or internal qualities of people and objects.

Han har dårlig rygg / stor nese / god samvittighet.
→ He has a bad back / a big nose / a good conscience.

Katten har kort pels.
→ The cat has short fur.

Han var kledd i hvit skjorte.
→ He was dressed in a white shirt.

Olav kjøper ikke ny dress.
→ Olav is not buying a new suit.

Idiomatic phrases

Norsk English
i godt humør in a good mood
ha hastverk be in a hurry
ha feber have a fever
ha rett til å … have the right to …
på avstand at a distance
i nødsfall in an emergency