Well, is a little difficult even for me that i'm a native speaker but i try to explain you. "Io", "me" and "mi" are personal pronouns "Io" is used as subject "me" and "mi" ( are so-called "forme oblique" ) are used as complement "me" is called "voce tonica" and generally follows the verb: "hai chiamato me" "mi" is called "voce atonica" which usually preceeds the verb:"mi hai chiamato" You can see this for the grammar rules: http://grammatica-italiana.dossier.net/grammatica-italiana-09.htm And here for some examples http://www.wordreference.com/iten/Mi http://www.wordreference.com/iten/Me
Yes the sense is this. But in your specific example is better "Grazie" and "Ti ringrazio" "Grazie a te" you can use in reply to a "Grazie": your reply can be: Grazie a te.
Same question: niente per me sounds good to me but I don't know whether it'd be fine. I have a feeling like 'niente'=nothing, 'nessuna'=none of them, but I'm not sure.
Yes, niente is nothing, nessuno/a is none, it kind of depends on the sentence. Here both make sense ;)
If the "nothing" was ,say, an apple would nessuna end in an 'a' or are nothings always masculine?
Not necessarly. It could be implied any word as long is femenine (gender)
No not really - in this case it would be like saying 'none for me' in English. The word 'none' doesn't imply 'thing'
this is touched on in some of the comments, but why is "nessuno per me" an incorrect answer?
What I say every time someone doles out cookies... Up vote if you are also gluten intolerant (:
And down-vote if you're not?
Someone is nice enough to bring cookies, and you complain because they didn't think to bring some gluten-free free cookies for you?
Next time, take your own cookies.