"This banana is good."
Translation:هٰذِهِ الْموزة طَيِّبة.
hadhaa هذا refers to masculine while hadhihi هذه is to feminine. How to differentiate between masculine and feminine is quiet tricky. Mostly nouns with ta marbuTa "ة" ending are feminine (such as "موزة") whereas the nouns without "ة" are masculine (like as "موز").
But, other nouns whether those are masculine or feminine should be memorised (irregular) -- or they have another signs. For example, "هذه رِجْل = this leg" is feminine even though it's without "ة".
(1) An interesting question! But, I still have no idea (except both words were from Ancient Arabic). Hope I'll find something in the future. Inshaa Allaah.
(2) Yes, you're correct! You may read some comments in Duolingo that say like that. But, I think it is just for the simplification, because actually, it is more complex.
For example, parts of bodies (like العنق, neck) can be applied as feminine or masculine. Also, some Classic Arab tribe (ie. Bani Abi Asad) had different opinions whether part of fingers are feminine or masculine.
We can read the Arabic version of this article: https://m.facebook.com/story.php?story_fbid=405201222959101&id=330296380449586
Here, almost all parts of bodies are explained. But, don't count too much on its English translation because it will confuse you. :)