[SOLVED] Sorting by modification date

How to do it?
I can do it adding
if( have_posts() ) :
// begin of my code >>
if (is_main_query()) :
query_posts($query_string.'orderby=modified');
endif;
// << end of my code
into begin of bfa_loop function (montezuma/includes/template_tags.php file)
but in this case all posts are shown each time not depend on if I select some tag or category. in correct order, but I don't need them all in those cases.