Forms :: One Combo Box Based On Another Combo Box And Text Box Uses

I have been creating a form, based on only one table. Here I am displaying data as text boxes and subfrom from the same table. At the beginning, I was interested in controlling the display of the data according to the combo box (in this case is the PO number). Now, I would like to add another combox box which is the year (I have a column with the date, and also I have a column that shows only the years digits in my table) Also, I have a subform that display the data from the same table specifics records that I want. It means that I want to pick the year first, and then in the combox box of po number shows only option of that year, and hence the text boxes and subforms change accordingly to the two combox boxes.