Is it Main or Mein (I) i have also seen it mai or mei? Does it really matter? standard way is to write 'main' मैं (u can see it in official transliteration and in movies) but it doesnt matter, as far as u can understand it means I 'mein' is the standard way to write (in) में. I urge u to stick with standard form. it doesnt matter by the way because u are not writing it wrong in devanagari. but still, it is better to follow standard practice. Tum or Aap (you) Hum (we) Veh or ve (they) Also when i am talking about myself do I use hoon? Ex; Main Khata hoon? Correct. u will say 'khaati', as u are a female. For you,we,they I use hain? yes! 'hain' is like 'are' Ex: (Aap khata hain is wrong) it should be, aap khate hain, Hum bolte hain, Veh chalte hain absolutely correct. when u say tum for 'you', instead of hain, u have to use 'ho', aap khaate hain. will be, tum khate ho similarly, tum chalte ho tum jaate ho in colloquial language, we use 'ho' with 'aap' as well. aap khate ho aap chalte ho aap jaate ho u shoul know this but stick with standard i.e. aap ....hain .

I agree with Tushar and would like to add that the best way to avoid confusion is to go with the Hindi script Devanagari. So मैं will be मैं no matter how it is spelled in English (main, mai, mei, mein). The native speakers of Hindi use different spellings in English (depends on each person) for Hindi words but it does not matter as the native speakers understand what is being said or written; as we don't focus on English spellings when the words are from Hindi. Hope it makes sense :D As for Devanagari script, just like I tell my students, it looks difficult but once you start learning it you will find it very interesting. And with regular practice it will become easy to read and write in Devanagari.