Method 1 - Redirect your main domain to a subfolder on that domain:
RewriteEngine On
# Change YourDomain.com to be your main domain.
RewriteCond %{HTTP_HOST} ^(www.)?YourDomain.com$
# Change 'subdirectory' to be the directory you will use for your main domain
RewriteRule ^(/)?$ subdirectory [L]
Method 2 - Show the content of a subdirectory while you are on your main domain:
RewriteEngine on
# Change YourDomain.com to be your main domain.
RewriteCond %{HTTP_HOST} ^(www.)?YourDomain.com$
# Change 'subdirectory' to be the directory you will use for your main domain.
RewriteCond %{REQUEST_URI} !^/subdirectory/
# Don't change the following two lines.
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
# Change 'subdirectory' to be the directory you will use for your main domain.
RewriteRule ^(.*)$ /subdirectory/$1
# Change YourDomain.com to be your main domain again.
# Change 'subdirectory' to be the directory you will use for your main domain
# followed by / then the main file for your site, index.php, index.html, etc.
RewriteCond %{HTTP_HOST} ^(www.)?YourDomain.com$
RewriteRule ^(/)?$ subdirectory/index.html [L]
