Review the differences between an abstract class and a base class. Review PHP constants.
Review how to call the correct base Class. Review Drupal core's file pathing to kernel tests.
Review the difference between how Drupal 8 forms retrieve data as opposed to Drupal 7.
Review Drupal Api docs in particular forms, caching, services and plugins.
Review Drupal Coding Standards such as line spacing between @doc blocks. Review PHP constants.
Review admin logs and status reports. Review how to write to the logging api.
Review SQL injection and more specificly examples of SQL injection syntax.