Who = quien
That = eso, esa, ese, tambien significa "que"
Who is doing your work now?
Who can see what are you doing?
Who is the best football player in the world?
Who must pay for the book tomorrow?
Who has to clean the livingroom at night?
I have to do that because is my obligation
I can fix that ice-cream machine now
If you are that, you must be the worst
those are the thing that you don't have to do
We could pay for that, but we don't have enough money.