There are no rules without exceptions….
There are some exceptions from the main rules. One on them is about masculine nouns that end in “er” in indefinite form singular. These nouns have a different declension in plural.
| SINGULAR | PLURAL | ||
| Indefinite | Definite | Indefinite | Definite |
| Regular | |||
| en gutt | gutten | gutter | guttene |
| Exception | |||
| en politiker | politikeren | politikere | politikerne |