This question has been answered by markh. See the first response.
RewriteEngine On
RewriteBase /
?
RewriteRule ^beboer/fora/thread/([0-9]+)/(.*)$ beboer/fora/?action=thread&thread=$1 [L,QSA] RewriteRule ^beboer/fora/u/(.+)$ beboer/fora/?action=user&user=$1 [L,QSA] RewriteRule ^beboer/fora/board/([0-9]+)/(.*)$ beboer/fora/?action=board&board=$1 [L,QSA] RewriteRule ^beboer/fora/category/([0-9]+)/(.*)$ beboer/fora/?category=$1 [L,QSA] RewriteRule ^beboer/fora/(.+)$ beboer/fora/?action=$1 [L,QSA] RewriteRule ^beboer/fora/(.+)/$ beboer/fora/?action=$1 [L,QSA]