.htaccess redirect main domain?

Question

How can I redirect requests to mydomain.tld/somepage.ext to mydomain.tld/mydomain/somepage.ext? I have subdomains like subdomain.mydomain.tld that I don't want to be affected by this.

I can't seem to get it to work right. I'm trying this:

RewriteEngine on

RewriteCond %{HTTP_HOST} mysite.com
RewriteCond %{REQUEST_URI} !^/mysite
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.*) /mysite/$1

But it isn't redirecting anything at all.

I also want to exclude one or two folders from this rule.

Answer 1

This seems to be working...

RewriteEngine on

RewriteCond %{HTTP_HOST} mysite\.com$ [NC]
RewriteCond %{REQUEST_URI} !^/mysite/         # but I don't see why this line is needed when I've used [L] below
RewriteCond %{REQUEST_URI} !^/ignored_folder/
RewriteRule .* /mysite/$0 [QSA,L]

At least for now.