Template structure
Template hierarchy
Template tags, conditional tags
Taxonomies, terms and post formats
Child theme concept
The Loop
Multiple and nested loops (wp_reset_postdata, wp_reset_query, rewind_posts)
Setting up and retrieving post data manually (setup_postdata, get_posts etc.)
Global variables
Hooks: actions and filters
wp_register_* and wp_enqueue_*
WP_Query. Properties and methods
Using $wpdb for database queries
WordPress database structure
HTTP API
WordPress coding standards
Data validation and filtration (sanitize_*, esc_* etc.)
Custom\meta fields
I18n (text domains, translatable strings etc.)
Settings API
Widgets API
Shortcodes API
Plugin API (hooks, installation, activation)
Post meta & user meta values
Routing (rewrite rules, permalinks, custom url structures etc.)
Debuggin tools
wp_cron (deferred, repeatable events)
Pluggable functions
Plugins DB management (dbDelta())
Drop-ins
Update services
Customization API
Queries customization via hooks