Why Mysql Insert Query Returns False

Some days ago I face a problem;  which is my mysql insert query retun false. So I need to dubug the query and wanna to know which problem is here. This help me very much:-

trigger_error(mysql_error());

to debug the query.  I think sometime you need help with that.

$sql = "here is you mysql query";
mysql_query($sql) or trigger_error(mysql_error()." in ".$sql);