"Activating" a form is what the user does when he/she clicks on it. I guess you mean you want the subform to become visible.
An "object in a field" would be an OLE object in a database field. I guess you mean a form control instead of a field, but what do you mean by an object in a form control? Maybe a particular value from a combo box?
To make a subform visible or invisible depending on what value is chosen in a combo box:
In Design View on the main form, right click on the On Update property for the combo box. Choose Build... and enter the following in the module window:
Private Sub cboMyComboBox_AfterUpdate()
If cboMyComboBox = "<the value that shows the subform>" Then
sbfMySubformControl.Visible = True
Else
sbfMySubformControl.Visible = False
End If
End Sub Rick Sprague