when im going to index.php?showforum=2 its not redirecting me to forum-2.html,
when im going to forum-1.html i see the content of index.php?showforum=1
No this is expected. The whole point is that your urls (the ones you use in your links) are in the 'short' format. This is what everybody sees. The 'long' url should still work, after all this is what the server sees with the short version (rewrite).