iNotes customization does not have much to do with forms and subforms. Almost the complete code for iNotes comes from the iNotes\FormsXX.nsf that comes with your Version of Domino.
Customize iNotes mostly is editing JavaScript- Code within Forms / Subforms of this Formsxx.nsf.
Use this link as a starting point or google for "iNotes Customization"
Best practice is to use a forms_x.nsf (this time the x in the name is not a placeholder, but the real name of the database) to hold your custom code, but all of that can be read following the above link.
Just remember: customizing iNotes does not have much to do with "classic" Notes development.