How to get the untranslate original string? In a form, there a submit button:

  $form['previous'] = array(
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Modify')
  );


In the form _submit function:

  switch($form_state['values']['op']) {
    case 'Modify' :
      // ...
      break;


the case 'Continue' will not match if Modify is translated. How can I always get the original untranslated Modify string for the sake of string comparison?
If there are multiple submit buttons on the form, I always prefer to have different submit functions for them. In your case I'd go for something like :

  $form['previous'] = array(
    '#type' =&gt; 'submit',
    '#value' =&gt; t('Modify'),
    '#submit' =&gt; array('_my_function_to_go_to_previous_function'),
  );


and implement the logic inside the submit:

function _my_function_to_go_to_previous_function($form, &amp;$form_state) {
  // Logic to go to previous step.
}


In this way you'll have more control over whatever you do. Works well with multistep forms.
You can also implement separate #validate function per button too.
Note that the $form_state parameter is passed by reference, which would mean you don't have to return anything from the submit function. Just manipulate the values inside the variable as per requirement, and then call

$form_state['rebuild'] = TRUE;


inside the submit function, so that the form rebuilds itself with the new values.