Ответ: Did she fall off a horse yesterday? No, she didn't. He fell off his bike.
Составляем вопрос, используя вспомогательный глагол Did для прошедшего времени (Past Simple) и информацию из условия: fall off a horse / yesterday.
Did she fall off a horse yesterday?
Отвечаем отрицательно, используя no/ his bike из условия. Составляем отрицательный ответ: No, she didn't. He fell off his bike.
Ответ: Did she fall off a horse yesterday? No, she didn't. He fell off his bike.