301 Redirect all page and post urls from .html to /

I have a current site where all URLs end with .html.

I created a new site and the URLs are pretty much the same but without .html.

I’ve been trying codes found here in my .htaccess file and most seem to cause Internal Server Error.

http://example.com/page1.html to http://example.com/page1/
http://example.com/page1/page2.html to http://example.com/page1/page2/

My current .htaccess file code is:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress