.htaccess issues

When trying to change WPFC settings, I’m told that .htaccess is not writable and to paste the replacement manually, not a problem but when I do so the replacement is distinctly different from the original, and actually shorter. When updated, other functions on the site stop working.

I’d like to make the change manually, to activate “Clear cache files when a post or page is updated” – can you tell me what the correct change to .htaccess is for this?

Also, in spite of not changing .htaccess, the above option has become selected in admin, will it have any effect, or is this shown in error?