Use the indefinite form of the noun without the article en/ett when:

•  the verbs vara or bli are used to express nationality or profession (Jag ska bli lärare)
• the noun is preceded by the pronouns varje, varenda, nästa or samma, or a numeral or quantifier
• it is preceded by a possessive form
• it is an abstract word or a mass word

The definite form of the noun is used when:

• the noun has been mentioned before, or if it is generally known or familiar from the situation.
• it is preceded by den/det/de här, den/det/de där, hela or förra (Åt du hela kanelbullen?)

Note:

förra året = last year

nästa år = next year

som lärare = as a teacher

som en lärare = like a teacher

Quantifiers

Quantifiers are chosen based on the countable nouns (e.g. många katter) and uncountable nouns (e.g. mycket vatten).

Quantifiers with countable nouns:

The noun takes the plural indefinite form after these quantifiers (en saft, saften, safter, safterna):

Nuförtiden innehåller allt färre safter socker.

These days, fewer and fewer juices contain sugar.

Quantifiers with uncountable nouns:

Uncountable nouns don’t take the plural forms, so the noun is in the singular indefinite form after these quantifiers (mjöl, mjölet):

Använd mindre mjöl nästa gång.

Use less flour next time.