参考主题WordPress自带主题， twentytwentyone，在文章模版下面，有个函数 ：

<?php comments_template(); ?>

这个函数会引用主题下的 comments.php，在该文件的底部有个函数 comment_form 函数，就是用来生成表单的。

//Declare Vars
$comment_send = 'Send';
$comment_reply = 'Leave a Message';
$comment_reply_to = 'Reply';
 
$comment_author = 'Name';
$comment_email = 'E-Mail';
$comment_body = 'Comment';
$comment_url = 'Website';
$comment_cookies_1 = ' By commenting you accept the';
$comment_cookies_2 = ' Privacy Policy';
 
$comment_before = 'Registration isn\'t required.';
 
$comment_cancel = 'Cancel Reply';
 
//Array
$comments_args = array(
    //Define Fields
    'fields' => array(
        //Author field
        'author' => '<p class="comment-form-author"><br /><input id="author" name="author" aria-required="true" placeholder="' . $comment_author .'"></input></p>',
        //Email Field
        'email' => '<p class="comment-form-email"><br /><input id="email" name="email" placeholder="' . $comment_email .'"></input></p>',
        //URL Field
        'url' => '<p class="comment-form-url"><br /><input id="url" name="url" placeholder="' . $comment_url .'"></input></p>',
        //Cookies
        'cookies' => '<input type="checkbox" required>' . $comment_cookies_1 . '<a href="' . get_privacy_policy_url() . '">' . $comment_cookies_2 . '</a>',
    ),
    // Change the title of send button
    'label_submit' => __( $comment_send ),
    // Change the title of the reply section
    'title_reply' => __( $comment_reply ),
    // Change the title of the reply section
    'title_reply_to' => __( $comment_reply_to ),
    //Cancel Reply Text
    'cancel_reply_link' => __( $comment_cancel ),
    // Redefine your own textarea (the comment body).
    'comment_field' => '<p class="comment-form-comment"><br /><textarea id="comment" name="comment" aria-required="true" placeholder="' . $comment_body .'"></textarea></p>',
    //Message Before Comment
    'comment_notes_before' => __( $comment_before),
    // Remove "Text or HTML to be displayed after the set of comment fields".
    'comment_notes_after' => '',
    //Submit Button ID
    'id_submit' => __( 'comment-submit' ),
);
comment_form( $comments_args );

另外列出一些，评论表单钩子:

comment_form_before
comment_form_must_log_in_after
comment_form_top
comment_form_logged_in_after
comment_notes_before
comment_form_before_fields
comment_form_field_{$name}
comment_form_after_fields
comment_form_field_comment
comment_form (action hook)
comment_form_after
comment_form_comments_closed

在主题的 functions.php 中添加：

function add_some_text() {
    echo '欢迎欢迎';
}
// 在默认字段（前面说的姓名、邮箱和网址）的下面添加字段
add_filter('comment_form_after_fields', 'add_some_text');
// 在已登录下面添加字段（因为用户登录后，是没有默认上面三个字段的），所以要使用这个钩子插入内容
add_filter('comment_form_logged_in_after', 'add_some_text');

参考：https://developer.wordpress.org/reference/functions/comment_form/