Customize the Drupal user forms (login & register)

customize_user_form.php

<?php

/**
 * Implements hook_user_login
 */
function MODULE_form_user_login_alter(&$form, &$form_state) {
  $form['already_customer'] = array(
    '#type' => 'markup',
    '#markup' => '<h3>' . t('I am already a customer') . '</h3>',
    '#weight' => -10,
  );
  $form['forgot_password'] = array(
    '#type' => 'markup',
    '#markup' => '<div class="forgot-password"><p>' . l(t('Forgot password?'), 'user/password', array('attributes' => array('title' => t('Forgot password?')))) . '</p></div>',
    '#weight' => 20,
  );
}

/**
 * Implements hook_user_register_form
 */
function MODULE_form_user_register_form_alter(&$form, &$form_state) {
  $form['new_customer'] = array(
    '#type' => 'markup',
    '#markup' => '<h3>' . t('New here?') . '</h3>',
    '#weight' => 0,
  );
}

Thanks <3
it also worked on THEME FILE your_theme_name.theme