Subject: How do you write formulas like Na2Cl for oxidation # and elements

In class we are learning about oxidation numbers and everyone is writing 
formulas for certain elements. A simple one I understand is H20, however I 
don't understand anything beyond that. What causes an element to have a 
subscript of 2 or 3, and how do you know what to put above the element 
symbol such as 3+. I know that has to do with electrons, but it still 
doesn't make sense!

Re: How do you write formulas like Na2Cl for oxidation # and elements