Adding logic to tell Wordpress to prepend `/articles/` to Posts only

rewrite-rules.php

function add_rewrite_rules($wp_rewrite) {
  $new_rules = array(
    'articles/(.+?)/?$' => 'index.php?post_type=post&name='. $wp_rewrite->preg_index(1),
  );

  $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}

add_action('generate_rewrite_rules', 'add_rewrite_rules');

function change_blog_links($post_link, $id=0) {
  $post = get_post($id);

  if(is_object($post) && $post->post_type == 'post'){
    if ($post->post_status == "publish") {
      return home_url('/articles/'. $post->post_name.'/');
    }
    if (is_admin()) {
      return home_url('/?p='. $post->ID . '&preview=true');
    }
  }

  return $post_link;
}

add_filter('post_link', 'change_blog_links', 1, 3);

// ht Fury on Stack Overflow for the original version of this
// see: https://wordpress.stackexchange.com/questions/52471/permalinks-question-adding-a-prefix-only-in-front-of-the-posts/63895#63895

View the related post for this code snippet, which discussed safely changing URL structure at the LoudNoises blog: https://www.loudnoises.us/blog/the-toe-dip-method-for-changing-url-structure