RewriteEngine on
RewriteRule f[\d]+/.+-([\d]+)/index([\d]+).html showthread.php?t=$1&page=$2 [NC,L]
RewriteRule f[\d]+/.+-([\d]+)/ showthread.php?t=$1 [NC,L]
RewriteRule f([\d]+)/index([\d]+).html forumdisplay.php?f=$1&page=$2 [NC,L]
RewriteRule f([\d]+)/ forumdisplay.php?f=$1 [NC,L]