Drupal generates form by fields' setings

gistfile1.php

<?php
  foreach ($fields as $field_name) {
    $field    = field_info_field($field_name);
    $instance = field_info_instance('node', $field_name, $field['bundles']['node'][0]);
    $form_field = field_default_form('node', null, $field, $instance, LANGUAGE_NONE, array(), $form, $form_state);
    $form += (array) $form_field;
  }