can someone please post a simple working example of form api module?

Plagiarism, a great force for progress. I hope that the code below will help you. I lifted it out of one of my own modules (not contributed due to licensing issues in one of the components. It involves the three main bits you need for forms. The titles_admin function builds the form, the theme_titles_admin_form function takes care of sticking some of the fields into a table, and the titles_admin_form_submit function takes care of the actions executed after the user presses "Submit".

function titles_admin_form_submit($form_id, $form_values) {
  $edit = $_POST['edit'];

    $result = db_query('DELETE FROM {titles_settings}');
    for ($i = 0; $i < $form_values['titles_count']; $i++) {
      if ($edit["titles_display_name_$i"] != '') {
        if ($edit["titles_weight_$i"] == '') {$edit["titles_weight_$i"] = 0;}
        if ($edit["titles_days_up_$i"] == '') {$edit["titles_days_up_$i"] = 0;}
        db_query("INSERT INTO {titles_settings} (uid, display_name, which_column, weight, days_up) VALUES ('%d', '%s', '%d', '%d', '%d')", $edit["titles_uid_$i"], $edit["titles_display_name_$i"],$edit["titles_which_column_$i"],$edit["titles_weight_$i"], $edit["titles_days_up_$i"]);
        }
      }
    variable_set('titles_sticky_title', $edit["titles_sticky_title"]);
    variable_set('titles_sticky_days_up', $edit["titles_sticky_days_up"]);
    drupal_set_message(t('The configuration has been saved'));

}

function theme_titles_admin_form($form) {

  $header = array(t('User name'), t('Display name'), t('Column'), t('Weight'), t('Days on front'));
  $rows = array();
  $count = $form['titles_count']['#value'];
  $output = form_render($form['titles_count']);
  for ($i = 0; $i < $count; $i++) {
    $row = array();
    $output .= form_render($form['users']["titles_uid_$i"]);
    $row[] = $form['users']['titles_name_'.$i]['#default_value'];
    $row[] = form_render($form['users']['titles_display_name_'.$i]);
    $row[] = form_render($form['users']["titles_which_column_$i"]);
    $row[] = form_render($form['users']["titles_weight_$i"]);
    $row[] = form_render($form['users']["titles_days_up_$i"]);
    $rows[] = $row;

    }
  $output .= theme_table($header, $rows);
  $output .= form_render($form['titles_sticky_title']);
  $output .= form_render($form['titles_sticky_days_up']);
  $output .= form_render($form['titles_press_button']);
  return $output;
}

function titles_admin() {
/* Settings to determine
   - what will be the display name for each user to be published
     (and whether all users will be published or not)
   - whether to place titles for a user in left, middle, or right column
   - order in which users are to be published
*/
  $form = array();
// Set up the form to display settings
  $result = db_query('SELECT u.uid, name, display_name, which_column, weight, days_up FROM {users} u left outer join {titles_settings} t on u.uid=t.uid where u.uid > 0 order by name');
  $count = db_num_rows($result);
  $form['titles_count'] = array(
	'#type' => 'hidden',
	'#value' => $count,
	);
  for ($i = 0; $i < $count; $i++) {
    $user_setting = db_fetch_object($result);
    foreach (array('uid', 'name', 'display_name', 'which_column', 'weight', 'days_up') as $field) {
      switch ($field) {
        case 'uid':
	    $form['users']["titles_uid_$i"] = array(
		'#type' => 'hidden',
		'#value' => $user_setting->uid,
		);
	    break;
        case 'name':
	    $form['users']['titles_'.$field.'_'.$i] = array(
		'#type' => 'textfield',
		'#title' => '',
		'#default_value' => $user_setting->name,
		'#size' => 15,
		'#maxlength' => 90,
		);
	    break;
        case 'display_name':
	    $form['users']['titles_'.$field.'_'.$i] = array(
		'#type' => 'textfield',
		'#title' => '',
		'#default_value' => $user_setting->display_name,
		'#size' => 15,
		'#maxlength' => 90,
		);
	    break;
        case 'which_column':
	    $form['users']['titles_'.$field.'_'.$i] = array(
		'#type' => 'select',
		'#title' => '',
		'#default_value' => $user_setting->which_column,
		'#options' => array(0=>'Left', 1=>'Middle', 2=>'Right'),
		);
            break;
        case 'weight':
	    $form['users']['titles_'.$field.'_'.$i] = array(
		'#type' => 'textfield',
		'#title' => '',
		'#default_value' => $user_setting->weight,
		'#size' => 2,
		'#maxlength' => 2,
		);
            break;
        case 'days_up':
	    $form['users']['titles_'.$field.'_'.$i] = array(
		'#type' => 'textfield',
		'#title' => '',
		'#default_value' => $user_setting->days_up,
		'#size' => 2,
		'#maxlength' => 2,
		);
            break;
       }
     }
   }

   $form['titles_sticky_title'] = array(
	'#type' => 'textfield',
	'#title' => t('Sticky articles title'),
	'#default_value' => variable_get("titles_sticky_title", 'Stop press'),
	'#size' => 100,
	'#maxlength' => 100,
	'#description' => 'Title for the top box containing sticky articles',
	'#attributes' => NULL,
	'#required' => TRUE,
	);
   $form['titles_sticky_days_up'] = array(
	'#type' => 'textfield',
	'#title' => t('Sticky days up'),
	'#default_value' => variable_get("titles_sticky_days_up", '0'),
	'#size' => 2,
	'#maxlength' => 2,
	'#description' => 'Days that sticky articles remain sticky (and therefore displayed in top box)',
	'#attributes' => NULL,
	'#required' => TRUE,
	);
   $form['titles_press_button'] = array(
   	'#type' => 'submit',
	'#value' => t('Update'),
	);
   $output = drupal_get_form('titles_admin_form', $form);
   return $output;
}