Past Continius 1.Make the statements negative 1) Bob was writing a letter at 5 p.m
2)It was snowing when they returned home 3)The wind was blowing from the east 4)It was raining the whole day yesterday 5)The boys were talking when I saw them
6)Nick was writing for Mary when we saw them 7)Mary was singing when he came home 8)It was snowing hard when father returned home from the office 9)The car was moving quickly when something wrong happened 10)I was watching Pirates of the Caribbean at noon yesterday

Чтобы сделать данные утверждения в Past Continuous отрицательными, необходимо использовать конструкцию was not (сокращённо wasn't) или were not (сокращённо weren't) в зависимости от подлежащего, и далее добавлять основное действие в форме ing. Вот как это выглядит:

1. Bob was writing a letter at 5 p.m.
   → Bob was not writing a letter at 5 p.m.
   (Боб не писал письмо в 5 вечера.)

2. It was snowing when they returned home.
   → It was not snowing when they returned home.
   (Не шёл снег, когда они вернулись домой.)

3. The wind was blowing from the east.
   → The wind was not blowing from the east.
   (Ветер не дул с востока.)

4. It was raining the whole day yesterday.
   → It was not raining the whole day yesterday.
   (Дождь не шёл весь день вчера.)

5. The boys were talking when I saw them.
   → The boys were not talking when I saw them.
   (Мальчики не разговаривали, когда я их увидел.)

6. Nick was writing for Mary when we saw them.
   → Nick was not writing for Mary when we saw them.
   (Ник не писал для Мэри, когда мы их увидели.)

7. Mary was singing when he came home.
   → Mary was not singing when he came home.
   (Мэри не пела, когда он пришёл домой.)

8. It was snowing hard when father returned home from the office.
   → It was not snowing hard when father returned home from the office.
   (Снег не шёл сильно, когда отец вернулся домой из офиса.)

9. The car was moving quickly when something wrong happened.
   → The car was not moving quickly when something wrong happened.
   (Машина не двигалась быстро, когда что-то пошло не так.)

10. I was watching Pirates of the Caribbean at noon yesterday.
    → I was not watching Pirates of the Caribbean at noon yesterday.
    (Я не смотрел «Пиратов Карибского моря» вчера в полдень.)

Эти отрицательные предложения корректно отражают форму Past Continuous.