Try this, replacing only " domain.com" and "forumpath".
Code:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www [NC]
RewriteRule (.*) http://www.domain.com/forumpath/$1 [R=301,L]
RewriteRule ^forums.* index.php [L,NC]
RewriteRule ^post-([0-9]*).html&highlight=([a-zA-Z0-9]*) viewtopic.php?p=$1&highlight=$2 [L,NC]
RewriteRule ^post-([0-9]*).* viewtopic.php?p=$1 [L,NC]
RewriteRule ^view-poll([0-9]*)-([0-9]*)-([a-zA-Z]*).* viewtopic.php?t=$1&postdays=$2&postorder=$3&vote=viewresult [L,NC]
RewriteRule ^about([0-9]*).html&highlight=([a-zA-Z0-9]*) viewtopic.php?t=$1&highlight=$2 [L,NC]
RewriteRule ^about([0-9]*).html&view=newest viewtopic.php?t=$1&view=newest [L,NC]
RewriteRule ^about([0-9]*)-([0-9]*)-([a-zA-Z]*)-([0-9]*).* viewtopic.php?t=$1&postdays=$2&postorder=$3&start=$4 [L,NC]
RewriteRule ^about([0-9]*)-([0-9]*).* viewtopic.php?t=$1&start=$2 [L,NC]
RewriteRule ^about([0-9]*).* viewtopic.php?t=$1 [L,NC]
RewriteRule ^about([0-9]*).html viewtopic.php?t=$1&start=$2&postdays=$3&postorder=$4&highlight=$5 [L,NC]
RewriteRule ^mark-forum([0-9]*).html* viewforum.php?f=$1&mark=topics [L,NC]
RewriteRule ^updates-topic([0-9]*).html* viewtopic.php?t=$1&watch=topic [L,NC]
RewriteRule ^stop-updates-topic([0-9]*).html* viewtopic.php?t=$1&unwatch=topic [L,NC]
RewriteRule ^forum-([0-9]*).html viewforum.php?f=$1 [L,NC]
RewriteRule ^forum-([0-9]*).* viewforum.php?f=$1 [L,NC]
RewriteRule ^topic-([0-9]*)-([0-9]*)-([0-9]*).* viewforum.php?f=$1&topicdays=$2&start=$3 [L,NC]
RewriteRule ^ptopic([0-9]*).* viewtopic.php?t=$1&view=previous [L,NC]
RewriteRule ^ntopic([0-9]*).* viewtopic.php?t=$1&view=next [L,NC]