The IF statement has both a TRUE and a FALSE argument. When you nest them, the TRUE/FALSE argument for each nested IF may not be right next to each other.
Work from the inside out (starting with the inner-most IF).
Can you please just list out all the conditions/criteria in plain English for us, describing what should happen in each instance, and what the order should be?
I find it is much easier to write the formula based on the conditions, then try to guess what the user wants to happen from errant formulas.