RewriteEngine on

# index
RewriteRule ^index.html$    index.php

# board
RewriteRule ^board_([0-9]+)\.html$   board.php?id=$1

# board with page
RewriteRule ^board_([0-9]+)_([0-9]+)\.html$   board.php?id=$1&page=$2

# topic
RewriteRule ^topic_([0-9]+)\.html$   topic.php?id=$1

# topic with page
RewriteRule ^topic_([0-9]+)_([0-9]+)\.html$   topic.php?id=$1&page=$2