Password hint for protected page

So, what’s your question? Are you asking how you should modify the code you provided in order to have it display an error?

Try something like:

<?php
function fb_the_password_form() {
global $post;
$label = 'pwbox-'.(empty($post->ID) ? rand() : $post->ID);
$output = '<form action="' . get_option('siteurl') . '/wp-pass.php" method="post">';
if( isset( $_REQUEST['post_password'] ) ) {
    $output .= '<p>' . __("The password you entered was incorrect. Please try again:") . '</p>';
} else {
    $output .= '<p>' . __("My post is password protected. Please ask me for a password:") . '</p>';
}
$output .= '<p><label for="' . $label . '">' . __("Password") . ' <input name="post_password" id="' . $label . '" type="password" size="20" /></label> <input type="submit" name="Submit" value="' . esc_attr__("Submit") . '" /></p>
</form>';
return $output;
}
add_filter('the_password_form', 'fb_the_password_form');
?>

That code should check to see if the form was already submitted (by checking to see if the value post_password form field was sent already) and output different intro text.

I forgot that WordPress uses an intermediate page in between submitting the form and returning to the protected post/page.

I think you’ll need to use the referer instead of trying to check the submitted information.

You might try something like:

<?php
function fb_the_password_form() {
global $post;
$label = 'pwbox-'.(empty($post->ID) ? rand() : $post->ID);
$output = '<form action="' . get_option('siteurl') . '/wp-pass.php" method="post">';
$prepg = wp_get_referer();
if( false === $prepg || !stristr( $prepg, 'wp-pass.php' ) ) {
    $output .= '<p>' . __("My post is password protected. Please ask me for a password:") . '</p>';
} else {
    $output .= '<p>' . __("The password you entered was incorrect. Please try again:") . '</p>';
}
$output .= '<p><label for="' . $label . '">' . __("Password") . ' <input name="post_password" id="' . $label . '" type="password" size="20" /></label> <input type="submit" name="Submit" value="' . esc_attr__("Submit") . '" /></p>
</form>';
return $output;
}
add_filter('the_password_form', 'fb_the_password_form');
?>