Hi,

it's all done by Inheritance : Object Inheritance - _inh

1-never touch the code openerp : no editing files directly , no editing from the Web UI. the best way is a create your custom module.

2-if you create a custom module inherit from others modules , you do not lose your modifications if any module gets an upgrade.

3- if you want to add a field you can add this in a custom module no DB (ORM OpenERP do this)

4 - must be used i18n.

you can start with this article : How i can create module openerp 7 ?