Drupal 7 module and theme developers setting out to update their code to Drupal 8 can benefit a lot from mapping what they already know about Drupal 7 to the way that they accomplish the same thing in Drupal 8. This generally involves picking a Drupal 7 pattern and then browsing through change records, and using tools like Drupal Module Upgrader to help identify the Drupal 8 equivalent.
We also recommend taking the time to familiarize yourself with Drupal 8 features like events, plugins, and services -- at least enough to recognize which pattern is appropriate for different types of problems. This will help you spot instances where there either isn't a direct Drupal 7 to 8 path, or where Drupal 8 might provide an updated way of accomplishing something.
- Update a custom Drupal 7 module so that it works with Drupal 8
- Convert a PHPTemplate (.tpl.php) file to Twig (.html.twig)
Beware of any tutorials or documentation showing before/after comparisons with Drupal 7 code from before Drupal 8 was released. Drupal APIs continued to change up until it was released, though many of these older tutorials are still accurate. We recommend spending some time learning current best practices so that when you encounter specific solutions you'll have a better chance to correctly gauge their accuracy.