“Sin/si/sitt/sine” is only used when:
1) the clause has a third-person subject (e.g. han, hun, de, gutten, Per)
Example: Jonas spiser maten sin. (Jonas is eating his food.)
“Jonas” is a third-person subject.
2) this third-person subject is the “owner” of the object/person
Example: Arnhild vasker bilen sin. (Arnhild is washing her car.)
“Arnhild”, the third-person subject, is the owner of the car.
3) the object/person “owned” is not part of the subject of the clause
Example: Petter og moren hans¹ gikk en tur med hunden sin². (Petter and his mother went for a walk with their dog.)
¹”hans” is part of the subject. You could not use “sin” here.
²”sin” is not part of the subject. It refers back to the subject, Petter and his mother.
More Examples:
- Joakim og søsteren hans¹ snakker flytende engelsk med mora si².
(Joakim and his sister speak fluent English with their mother.)
¹”hans” must be used here because it is part of the subject of the clause: Joakim og søsteren hans. Sin cannot be used in the subject.
²“sin” means “their” and refers back to the subject: Joakim og søsteren hans
- Yngve sier at søstera hans¹ skulle vaske bilen sin².
(Yngve says that his sister should wash her car.)
¹”hans” must be used here because it is part of the subject of the subordinate clause beginning with “at”.
²“sin” refers back to the subject of the clause, the sister.