I'm no expert but I've noticed in a sentence such as this it would be, She gives me ""her"" car because they set up the who's car she gave with ""She gives"" - there is no one else or any place else (another person, car rental, car lot for example) mentioned in the sentence other than "she", we have to deduct she gives her car. There is no other information to tell us differently. Although su can mean he, she, it, you, there is no information to tell us it is anyone other than "her". Given more information, certainly "his" could be totally correct, it is not correct for this sentence. You'll see this kind of thing in other lesson sentences but even when it is totally grammatically correct, and tempting to want to change it up, stick with the information given in the sentence or risk losing hearts.