"She eats her sandwich."
Translation:Hon äter sin smörgås.
Henne = her (she is the object of action)
Hennes = her(s) (she is the owner of something)
Sin = referring back to the third person subject of a sentence
Hon äter sin smörgås = she is eating her own sandwich
Hon äter hennes smörgås = she is eating someone else's sandwich
You can't say "hon äter henne smörgås" though, because that doesn't make sense.