i have one forum . i want one topic for guests to let them post without registration (not for one forum) .
Is it posible ? How ?

In miniBB's bb_specials.php there is an array $regUsrForums where you need to put your forum's ID, and DISABLE $allForumsReg setting in setup_options.php. Then note the topic's ID which you would like to set up for guests, and in bb_plugins.php paste this code:

if($user_id==0 and in_array($forum, $regUsrForums) and $topic_id==TOPIC_ID) $regUsrForums=array();

this would work only for one forum if you have it... for many forums there would be other code which I have no time to look for now.

Paul ,
thanks .
i did it :
-in forum you can post NEW topic as guest .
-in old topics you need registration .

Hmm, ok then try to paste this code at the end of your language pack(s).