Translation:He gave his sister the apple.
Why is it "seiner Schwester" here? Is it because this is dative? http://german.about.com/library/blcase_dat.htm
I think so.
Er gab does not follow the list of endings shown in the introduction.
Hint say this is "He gave his sister his apple."
So far, the only consistent thing about these Präterit forms is that the main vowel is changed to "a".