Here is important reference/points to learn word press.
Function Reference – https://developer.wordpress.org/reference/
WordPress Coding Standards – https://make.wordpress.org/core/handbook/best-practices/coding-standards/
Data Validation – https://codex.wordpress.org/Data_Validation
Global Variables – https://codex.wordpress.org/Global_Variables
Post Types – https://codex.wordpress.org/Post_Types
Taxonomies- https://codex.wordpress.org/Taxonomies
Query Overview – https://codex.wordpress.org/Query_Overview
Plugin Developer Handbook- https://developer.wordpress.org/plugins/
Theme Developer Handbook – https://developer.wordpress.org/themes/
Detailed Overview of class WP_Query –Â https://codex.wordpress.org/Class_Reference/WP_Query
Custom Queries – https://codex.wordpress.org/Custom_Queries
Hooks – https://developer.wordpress.org/reference/hooks/
WP AJAX –Â https://codex.wordpress.org/AJAX_in_Plugins
Plugin API/Filter Reference – https://codex.wordpress.org/Plugin_API/Filter_Reference
Plugin API/Action Reference – https://codex.wordpress.org/Plugin_API/Action_Reference
Conditional Tags – https://codex.wordpress.org/Conditional_Tags
Templates – https://codex.wordpress.org/Templates
Template Hierarchy – https://developer.wordpress.org/themes/basics/template-hierarchy/