The genitive expresses ownership, to whom or what something belongs.

S-genitive

Ownership in Swedish is usually expressed by a structure called the s-genitive, but can sometimes also be expressed by alternative structures such as compounds and prepositional phrases. The s-genitive is formed by adding the genitive ending -s to the end of a proper noun or to the end of a noun in its definite form:

Lauras hund är under bordet.

Laura’s dog is under the table.

Lärarens nya bil var stor och röd.

The teacher’s new car was big and red.

The noun following the genitive form is always indefinite. If that noun takes an adjective, the adjective is used in its definite form (marked by the letter a):

familjens stora hus

Marias röda rum

ert fina ställe

Sin, sitt, sina

The pronouns sin, sitt, sina are used with the third person singular (han, hon, hen) and third person plural (de) when the owner is the subject of the same clause:

Han har inte korrekt kabel för att ladda sin telefon.

He doesn’t have the right cable to charge his phone.

Sin, sitt, sina inflect according to the gender and number of the noun, just like other possessive pronouns, e.g. min, mitt, mina.

Oskar hittade inte sina filer som han precis hade laddad ner.

Oskar couldn’t find his files that he had just downloaded.

Note:

Min granne tar väl hand om sitt barn.

My neighbour takes good care of his/her (own) child.

Min granne tar väl hand om hennes barn.

My neighbour takes good care of her (someone else’s) child.