If I understand you, you want to filter a combo or list box on a subform based on a value in a cotrol on the mainform. If so, then in the query behind your combo/list set the criteria for the relevent column to:
=Forms!mainformname!controlname
You should also do a Requery of the combo/kist. I would do it in the Gotfocus event:
Me!controlname.Requery.