How to use form field only once I'm looking for solution for following scenario in my D7 site.
I have form with 2 empty text fields. If user enters value in field 1 and submits form next time when he opens this form only field 2 will be available for input.
you can achieve it with some code in your custom module 

function yourmodule_form_alter(&amp;$form, &amp;$form_state, $form_id)
{
    if ($form_id == 'your_form_id' ) { // in your custom content type 
    if($form["nid"]["#value"]){  //when we are in edit form
      if(isset($form['field_your_field1']['und'][0]['#default_value']['value']) )
          hide($form['field_your_field1']);
     if(isset($form['field_your_field2']['und'][0]['#default_value']['value']) )
          hide($form['field_your_field2']);
    }
}
}