can I put an if statement in the where clause?

I have a form the shows the users transactions, I have two combo boxes, one for year, and one for month when they change the year and/or month it does a requery to match.

I was trying to figure out how to return all records, by putting an all in each of the combo box tables, but am unable to figure out how to get my query to return all.

maybe just better to create a history form that shows all?