- en gutt - a boy
- gutt + en --> gutten - the boy
In the second example, the definite singular is created by adding the indefinite article at the end of the word itself in the form of a suffix.
Han er en gutt. - He is a boy.
(You see a child on a street who happens to be a boy.)
Gutten spiser et smørbrød. - The boy is eating a sandwich.
(Marius, who is 8 years old, is eating a sandwich made by his grandma.)
The usage of indefinite (en, ei, et) and definite (word + -en, -a, -et) forms is similar to "a/an" and "the" in English, meaning that they determine something already familiar/specific (definite) or something vague/general (indefinite).