Welcome to the forum......
First terminology. A "subform" is a form embedded in another form. You don't "open" a subform (although a subform can be opened independently of the main form). It sounds like you just want to open a different form.
You didn't give names of object so I am going to make up names.
You have a table of countries named "tblCountries".
You have a main form named "frmMainForm"
On the main form you have a list box named "lstCountries".
The row source is a query based on the table "tblCountries".
The form you want to open after selecting a country is named "frmForm2"
The question now is "What does the form "frmForm2" do/display? What is the record source? Is it to create a new record? Edit a record?
Would you post an image of "frmForm2"?
The basic code to open a form would be
Code:
Private Sub lstCountries_DblClick(Cancel As Integer)
DoCmd.OpenForm "frmForm2"
End Sub
but this will change depending on what you want the form to do.
Maybe you could post your dB with a few records.......????