Why it is not "Jentas genseren", since there is "The girl's sweater"?
The sweater is already made definite when it's described as belonging to the girl. Thus we cannot add the define ending 'en' onto 'genser'. That would be like saying in English, "the girl's the sweater".
Is "Jenta sin" acceptable?
Yes, "Jenta sin genser" is, as of this reply, accepted by Duolingo.
Why is it hvit not hvitt?
“Hvitt” is the neuter form of “hvit”, while “hvit” is used for masculine and feminine. “Genser” is masculine.
why is en jentas genser er hvit marked wrong?
"jenta" is definite, so you can not add the indefinite article "en" in front of it.
Jentas genseren er hvit : why is it rejected by DuoLingo ? That English sentence means : the sweater of the girl is mine, so the sweater becomes genseren, no ?