Решение:
- В предложении "Helen didn't come to the party last night," Bob said., для преобразования прямой речи в косвенную, необходимо изменить 'didn't come' на 'hadn't come' (Past Perfect), 'last night' на 'the previous night' и добавить союз 'that'.
Ответ: Bob said that Helen hadn't come to the party the previous night.