Чуть подправила исходное предложение ("friend" вместо "friends").
Общий вопрос: Did my friend visit his grandparents last summer?
Специальный вопрос: When did my friend visit his grandparents?
Разделительный вопрос: Last summer my friend visited his grandparents, didn't he?
Отрицание: Last summer my friend didn't visit his grandparents.