If your case is just to add ONE other, as you describe, I would rather use "multiple choice" and activate the other option, because there the behaviour is how you wish. As soon as you write something in the other text box, the checkbox is automatically ticked by Limesurvey.
Of course this version does not work if you need more than one comment box. But for your example given, this is actually the correct question type and setting.
For the multipe choice with comments you need to first tick the checkbox and then write the comment, otherwise you will receive an error message. There you would have to create a workaround with Javascript or something.
Of course this version does not work if you need more than one comment box. But for your example given, this is actually the correct question type and setting.
For the multipe choice with comments you need to first tick the checkbox and then write the comment, otherwise you will receive an error message. There you would have to create a workaround with Javascript or something.