"I am drinking her tea."
Translation:Jeg drikker teen hennes.
Some possessive pronouns always end in an -s: hennes, hans, deres, dens, dets
While others never do: min, mitt, mine, din, ditt, dine, vår, vårt, våre
The ones with an -s are just personal pronouns with a genitive -s tagged onto them. They stay constant.
The ones without an -s are the ones that have to agree with the gender and number of the noun: min katt, mitt hus, mine barn
I'm brand new to Norsk, so this might be wrong, but.... I think you use sin/si/sitt/sine when it refers to the subject of the sentence. For this question, the subject is "Jeg", so "Jeg drikker teen hennes". But if it were "Hun" who is drinking her own tea, I think it would be "Hun drikker teen sin." Could anyone chime in and confirm/deny this explanation?