if you write on money is it still worth the amount or could it be accepted in a bank
It is still good if you write on it. The only way money is no longer good is if it is torn in two and you have less than half the bill.
It depends, where is it written on and how much is it written on?
It is still legal tender, but you could actually get scolded if someone in authority finds out you did it.
Yes. As long as you can tell it's a legal Federal Reserve Note you can use it.
as long as it's recognizable and has BOTH serial #'s.
Defacing currency will require it to be recycled. Take it to any bank, they will send it in to be recycled at the local Fed or Treasury office. They shred and burn them.