Why use t() function?

1-why to use t() function and not just ' print $variable ' ?

To make strings used in Drupal translatable. Every string in a t() function can be translated through the Drupal UI. That way people who use a Drupal site in a different language can adapt it to their needs.

2-why can't i use a variable in it ?

You can, but you have to do it through a placeholder like one of these:

$text = t("This is !name's website", array('!name' => $username));
$text = t("This is @name's website", array('@name' => $username));
$text = t("This is %name's website", array('%name' => $username));

See http://api.drupal.org/api/drupal/includes!bootstrap.inc/function/t/7

3-are function like "format_string($string, array $args = array())" & check_plain($text) & drupal_placeholder($text) are auto-include in it ?

It depends on what you use as a placeholder:

• !variable: Inserted as is. Use this for text that has already been sanitized.
• @variable: Escaped to HTML using check_plain(). Use this for anything displayed on a page on the site.
• %variable: Escaped as a placeholder for user-submitted content using drupal_placeholder(), which shows up as emphasized text.

See http://api.drupal.org/api/drupal/includes!bootstrap.inc/function/format_string/7

t() is mainly used to 'Translates a string to the current language or to a given language.'

To answer your first two questions, see http://api.drupal.org/api/drupal/includes%21bootstrap.inc/function/t/7

Also, simply using 'print $variable ' may cause security problem, see http://drupal.org/node/28984

These functions are auto inclued, and they could be used in code, depending these functions have been loaded. When drupal is still in bootstrap phase, some functions may not be available. For example, many drupal functions are not available inside hook_boot() http://api.drupal.org/api/drupal/modules%21system%21system.api.php/funct...