How to fix broken WordPress permalinks using .htaccess rewrite rules

You might run into permalink issues in WordPress installations which cause WordPress posts/pages to return “404 Not Found” error codes. In my case the default permalink setting “plain” worked fine until I switched over to “postname” (for SEO reasons) or any other permalink setting. This issue is most likely caused by installed WordPress plugins, which modify/redirect incoming http/https requests.

The WordPress plugin ReallySimpleSSL for instance, takes care about
changing the WordPress site URL / home URL to https and redirects all incoming requests to https as well.

ReallySimpleSSL point out on their homepage that using a .htaccess redirect “is a recommended setting, because the .htaccess redirect is slightly faster, and the WordPress [301] redirect might be less reliable in combination with caching“.

Following their advise the recommended setup should look like this:

If this option is enabled, ReallySimpleSSL modifies the .htaccess file somehow similar like this:

# BEGIN rlrssslReallySimpleSSL rsssl_version[3.2.6]
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule ^(.*)$ https://%{HTTP_HOST}/$1 [R=301,L]
</IfModule>
# END rlrssslReallySimpleSSL

So what does this mean particularly?

IfModule checks if an module called mod_rewrite.c is in place and installed correctly.
RewriteEngine enables the rewrite engine in general.
RewriteCond defines conditions under which rewriting takes place.
RewriteRule defines rules for the rewriting engine.

https://www.wpbeginner.com/wp-tutorials/how-to-fix-wordpress-posts-returning-404-error

UPDATE:

Just a side-note how WordPress handles a Multi-Site configuration:

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]