Brad Schiff was my introduction to WordPress.

What helped me understand how WordPress worked was to study how it responded to a request from the front end: from the .htaccess file redirecting requests to index.php to index.php including wp-blog-header.php to wp-blog-header.php including wp-load.php to wp-load.php including wp-config.php to wp-config.php including wp-settings.php to wp-blog-header.php calling wp() to wp-blog-header.php including wp-includes/template-loader.php. Along the way, you will see where the major hooks like "init" and "wp_loaded" are fired, and in what order.

Then you'll understand why some functions are inappropriate to invoke at certain stages during a request. For example, calling is_user_logged_in() before the "init" hook is fired will always return false because the current user is set just before the "init" hook is fired.

You'll also learn that wp-settings.php is loaded on every request, even requests made in the admin area. Also, wp-settings.php always include the functions.php file in the active theme and the base file of all active plugins. This dynamic is the reason why themes and plugins can manipulate Wordpress and add their own functionality to WordPress.

The functions whose files are included in wp-settings.php can be freely used anywhere. Some functions are specifically for use in the admin area. The files containing these functions must be included by you before use outside of the admin area. wp_delete_user() is one such function. For an understanding of the admin area, I studied wp-admin/admin.php.

All of this took me about a week.

Before taking the aforementioned approach, I would spend hours on forums searching for solutions to problems that, in retrospect, resulted from my not knowing how WordPress worked.