In form view, inside <notebook> tag you can add as many pages(TAB) by this syntax

 <notebook>
     <page string="Your Tab1">
         <field name="..."/>
     </page>  
     <page string="Your Tab2">
         <field name="..."/>
     </page>   
 </notebook>

hello Math

Here for you I'm adding new tab in "Quotations" of "sale" module,this is similar to your requirement

Example.py

from openerp import fields, models, api, _

class class_name(models.Model):

    _inherit = 'sale.order'
    <fields name="..."/> 

Example.xml

<record id="view_sale_order_tree" model="ir.ui.view">
    <field name="name">view.sale.order.tree</field>
    <field name="model">sale.order</field>
    <field name="inherit_id" ref="sale.view_order_form"/>
    <field name="arch" type="xml">

    <xpath expr="//page[@string='Other Information']" position="after">
        <page string="My_New_Tab">
            <field name='My_new_fields'/>
        </page>
    </xpath>

    </field>
</record> 

 

inside <notebook> tag, after <page string="Personal Information"> </page> put your new tab, <page string="My Tab"> </page>

You need to add a new page in the notebook in the View. Are you creating an own Addon/Module to alter the partner page?

In case of you do please take a look on that: You need to inherit from the partner view in maybe partner_view.xml:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <!--
    Partners Extension
  -->

    <record id="view_partner_property_form" model="ir.ui.view">
        <field name="name">res.partner.stock.property.form.inherit</field>
        <field name="model">res.partner</field>
        <field name="inherit_id" ref="product.view_partner_property_form"/>
        <field name="arch" type="xml">
            <notebook position="inside">
                <page string="My New Tab">
                    <field name="..."/>
                </page>
            </notebook>
        </field>
    </record>

  </data>
</openerp>

How can we set one tab or page as default, i.e., when i open the screen that that tab opens by default

For adding a new tab your xpath had to be like this

<xpath expr="//page[2]" position='after'>
1,2,3 depending the number of your tab in the view.