Well sorry for yet another confusing title, anyway, I just want to know which part of phpbb-seo package redirect forums/ to forum.html since I have just lost this functionality on my forum. In .htaccess I can find forum.html to forums/index.php but not vice versa.

Please help, thanks

I have already asked this:
Change fourm.html

Hope that is what you wanted to know

Thanks but that's not actually the problem, anyway let's continue the discussion there

All the redirecting is performed by the zero duplicate. The code doing this is in index.php.

The zero dupe only redirects example.com/phpBB/ (and example.com/phpBB/index.php) to example.com/phpBB/forum.html if you are using forum.html as you forum index url, but you could also use "nothing" and see example.com/phpBB/index.php and example.com/phpBB/forum.html redirected to example.com/phpBB/.

So, if you see example.com/phpBB/ being redirected to example.com/phpBB/forum.html, it may be because it is required for some option (such as virtual root) or because you have chosen it. In such case, the .htaccess generator should add the appropriate rewriterule to handle it. But it's not the rewriterules that will do the actual redirect.

++

Actually, my problem is it doesn't redirect to domain/forum.html if I goes to domain.com/forums/.

In the phpbb-seo setup file I got nothing like this: $this->seo_static['index'] = 'forum'; in .htaccess I got a rewriterule to rewrite domain.com/forum.html to domain.com/forums/index.php internally. Since the 301 redirection from forums to forum.html ain't in .htaccess, where could it be? I saw something in the index.php that does this, but as stated above I got nothing about index in phpbb-seo setup file.

Thanks for your help!

The zero duplicate indeed adds some code to do this in index.php :
index.php <= to forum.html
http://demo.phpbb-seo.net/index.php <= to /

Do grab its install and see if you did add everything, and make sure that it's activated in acp.