Options +FollowSymLinks
RewriteEngine on

RewriteRule ^buscar/$ index.php?a=buscar [L,NC]
RewriteRule ^(.*)/(.*)/(.*).html index.php?a=$1&id=$2&tag=$3 [L,NC]
RewriteRule ^(.*)/(.*)/(.*)/$ index.php?a=$1&id=$2&tag=$3 [L,NC]
RewriteRule ^(.*)/$ index.php?c=$1 [NC]

ErrorDocument 404 /index.php?c=error-404