How can I wrap my form in a table ?

here is my code:

ffunction my_module_my_form($form_state) {

  $form['description'] = array(
    '#type' => 'item',
    '#value' => t('Enter your cridentials:'),
  );
  $form['first'] = array(
    '#type' => 'textfield',
    '#title' => t('First name'),
  );
  $form['middle'] = array(
    '#type' => 'textfield',
    '#title' => t('Middle name'),
  );
  $form['last'] = array(
    '#type' => 'textfield',
    '#title' => t('Last name'),
  );
  
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Submit'),
  );
  return $form;
}

theme_my_module_my_form($form)
{
$output=drupal_render($form["description"]);
$output.='<table  class="my_module_form">';
$output.='<tr><td>'.drupal_render($form["first"]).'</td><td>'.drupal_render($form["middle"]).'</td><td>'.drupal_render($form["last"]).'</td></tr>';
$output.='<tr><td>'.drupal_render($form["submit"]).'</td></tr>';
$output.='</table>';
$output.=drupal_render($form);
return $output;
}

I don't understand. But theme_my_module_my_form() doen't work in drupal6!