Changing the recordsource and passing parameters

I have a form that has it's recordsource set to a query which includes the data from two tables.  The first table is a master table and the second is a detail table.  I am using an unbound form to ask the user which master record they want.  The form has the name & address information from the master table plus a subform for the detail records.  The navigation keys (custom) are in the subform.  All works well for editing and deleting but... when I need to add a detail record I run into trouble.  I know what mode I'm in because I pass openargs to the form.  How can I locate the master record that they want, pass key information to the form and go into add mode?   What about the link fields in the subform?  Can anyone help me?  Does anyone have any samples that do this?  I'm am very frustrated....HELP!