From where d7 render comment_form Can i know from where drupal core render comment_form Actually what i mean is what is the location of file which bear comment_form ..In my d7 folder , i am now testing it on localhost win7.I want to know it so that i can easily do change in comment form texts, do change in comment buttons, is this possible?
@rakibtg, To change the comment button text you are not supposed to change any comment module which is available in the core. Simply Never hack Core!!!

For your question, to change the comment button text, 

1.You need to write up a custom module with hook_form_alter,

  function mymodule_form_comment_form_alter(&amp;$form, &amp;$form_state, $form_id) {
     $form['actions']['submit']['#value'] =&gt; t('Post Comments');
 }