What should I write in the .htaccess file
to obtain
renaming url link for discussions and forums such as
thread-35353.html as here on mybb.com?
new installation of MyBB contain example file htaccess.txt in root folder.
You should look into this file as it answers your question.
If for some reason you dont have this file
you may download new installation package of MyBB from
http://mybb.com to obtain it.
You could get Google SEO instead.