<?php
! defined( 'ABSPATH' ) and exit;
/* Plugin Name: Redirect Main Site To Sub-Site */
function wpse66115_redirect_to_sub_site()
{
    if ( 'main-site' === $GLOBALS['wp']->request )
        exit( wp_redirect( 'http://www.example.com/main-site/sub-site/', 301 ) );
}
add_action( 'parse_request', 'wpse66115_redirect_to_sub_site' );