How come "she eats" would be वह खाता है, but "she eats the banana" is वह केले खाती है? Why is one "खाता" while the other is "खाती"?
She eats would be "vah khaati hai".
'Khaata' would be used if you were translating 'he eats.'
Hmm...why is it "the" banana? "The girl eats banana" was considered wrong, though I think I have seen this form of translation before on duolingo hindi....
You are correct. The system should accept "girl eats banana" because Hindi does not use any articles. Alternatively, for better English translation "The girl eats the banana." sounds better.
केला implies singular, therefore "a" may not be correct. Therefore, I am afraid "The girl eats A banana" will not be correct. Your sentence translates as..... laRki ek kela khati he.
Yes, but kela, the object is masculine. So it should be, “लड़की केला खाता हे।
It's always the subject which governs verb declension, exept for ने- and को- constructions, in which cases the object becomes the grammatical subject. And there's no form हे of the word होना.