I heard that this trended on twitter few days back.....
And led to some north india vs south India battle unnecessarily...
Tu is when you are close to somebody but when talking to that person it can be tu/tumhi.
Tumhi is equivalent to Aap.
Tum is Hindi addition which is in between tu and aap. 😃
For eg I call Lata Mangeshkar and Sachin Tendulkar as To/Ti... but ofcourse if I to meet them I will address them as tumhi.... The thing I feel attachment to them ... but will give appropriate respect.
Hindi speaking regions do not like tu... it's down grading for them... and so is yaar... I let go of this Mumbai Mumbaiya habit of saying yaar when I Shifted out of Maharashtra. It's scorned upon.