Ответ:
Общий вопрос
Had he walked the dog when the accident happened?
Разделительный вопрос
He had walked the dog when the accident happened, hadn't he?
Специальный вопрос
When had he walked the dog?
Альтернативный вопрос
Had he walked the dog or had sat in park when the accident happened ?
Вопрос к подлежащему
Who had walked the dog when the accident happened?