Thanks for the help.
Changing that line from true to false worked for getting the subdirectories set up. Now mainsite.com/forums opens but is clearly broken with no theme displaying. When I try to go to /forums/wp-admin I get a I get a redirect error. I was never prompted with new htaccess code.
Here is what is in there now….
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
# END WordPress