Redirect old homepage to the new one within the same site

Ideally, the blog subdomain would point to a different filesystem location (its own document root), then you could use a simple Redirect directive in its own .htaccess file in the root of the subdomain. For example:

Redirect 301 /

The Redirect directive is prefix-matching and everything after the match is copied onto the end of the target, so is redirected to

However, if both hostnames point to the same place (as would seem to be the case) then you will need to use mod_rewrite at the very top of the root .htaccess file. For example:

RewriteCond %{HTTP_HOST} ^blog\.(example\.com) [NC]
RewriteRule ^ https://www.%1/ratgeber%{REQUEST_URI} [R=301,L]

This specifically checks for the hostname in the preceding condition. The %1 backreference simply saves repetition having captured the domain name in the preceding condition.

The REQUEST_URI server variable already contains the slash prefix.

Always test first with a 302 (temporary) redirect to avoid potential caching issues. You should clear your browser cache before testing.

If you aren’t serving multiple domains (that could also have a blog subdomain) then you can make the rule entirely generic (to avoid hardcoding the domain name), to redirect from the blog subdomain to www (plus subdirectory). For example:

RewriteCond %{HTTP_HOST} ^blog\.(.+?)\.?$ [NC]