example of various uses of hook_form_alter()

um_common_form-alter.php

function um_common_form_alter(&$form, $form_state, $form_id) {
  // hide split teaser on node forms
  if (isset($form['#node']) && $form_id == $form['#node']->type .'_node_form') {
    $form['body_field']['teaser_include'] = array(
      '#type' => 'value',
      '#value' => TRUE,
    );
  }
  // user login block changes
  elseif(($form_id == 'user_login_block')) {
     $items = array();
        if (variable_get('user_register', 1)) {
           // change the user register path from "user/register" to "create-account" (node/8542)
           $items[] = l(t('Create account'), 'create-account', array('attributes' => array('title' => t('Create a free user account.'))));
        }
        $items[] = l(t('Forgot password?'), 'user/password', array('attributes' => array('title' => t('Request new password via e-mail.'))));
        unset($form['links']);
        $form['#suffix'] .= theme('item_list', $items);
     }
    // prefixing a form with javascript
    elseif($form_id == 'user_register') {
       $form['#prefix'] =
            '<script type="text/javascript">
                $(document).ready(function() {
                   $("#editrow-group :checkbox").attr("checked", "checked")
                });
             </script>';
        }
    // hiding some useless form elements
     elseif($form_id == 'user_profile_form') {
          $form['blog']['#access'] = FALSE;
                $form['theme_select']['#access'] = FALSE;
                $form['block']['#access'] = FALSE;
                $form['nodewords']['#access'] = FALSE;
        }
}