RewriteEngine On

RewriteRule ^f([0-9]+).*$ forums/$1 [L,R=301]
RewriteRule ^t(.*).html$ threads/$1 [L,R=301]
RewriteRule ^u(.*).*$ members/$1 [L,R=301] 
RewriteRule ^tags(.*).html$ tags/$1 [L,R=301]