See my above comment--every noun has a gender (feminine, masculine or neuter) and each gender has a different form set for the definite and indefinite versions:
- Feminine: ei bok - "a book"; boka - "the book"
- Masculine: en edderkopp - "a spider"; edderkoppen - "the spider"
- Neuter: et dyr - "an animal"; dyret - "the animal"
NB: feminine nouns are inconsistently treated. They all may used with the masculine forms, and it will vary based on dialect and region what is conventional. Many dialects have certain feminine words typically using the feminine forms, with others more often in the masculine form.