How to redirect root and only root via htaccess?

I want to redirect only my root to another url, but maintain all the /sub/directories where they belong (and redirect)

example:

mysite.com/1 redirects to somewhere mysite.com/admin opens a page

i want mysite.com/ to redirect to mysecondsite.com and only this with a 301 redirect using htaccess

Try this:

RewriteEngine on
RewriteCond %{HTTP_HOST} mysite\.com [NC]
RewriteCond %{REQUEST_URI} ^/$
Rewriterule ^(.*)$ http://mysecondsite.com/ [L,R=301]

If you don't need to check for the old domain (for example, if the directory where your .htaccess is placed is only used by the old domain) you can remove the second line.

If you mean you'd only like to redirect "/" to another domain, this will work:

RewriteEngine on
RewriteRule ^$ http://www.example.com/ [R=301,L]

This only matches the domain's root with nothing after it so it will only redirect the domain name without a filename specified.

This should work just fine:

RedirectMatch 301 ^/$ https://example.com/