You can filter a form(or report's) recordsource when calling it via the DoCmd.OpenForm method. It is the {wherecondition} property. The wherecondition is a SQL WHERE statement without the word 'where'. It works very similar to the criteria property of a query.
DoCmd.OpenForm "formaname", , , "[criteriafield] = 'some criteria'"
You can always concatenate something into the where clause. For example, if you want to limit the form you're opening to the current record of the form you're on use something like the following:
DoCmd.OpenForm "formname", , , "[primarykey] = " & Me.primarykey