4. My friends __________ (not to do) their homework at seven o'clock yesterday. They __________ (to play) volleyball.

4. My friends were not doing their homework at seven o'clock yesterday. They were playing volleyball.

В первом случае необходимо употребить Past Continuous в отрицательной форме, т.к. действие не происходило в определенное время в прошлом. Во втором случае - Past Continuous, т.к. действие происходило в определенное время в прошлом.

Ответ: My friends were not doing their homework at seven o'clock yesterday. They were playing volleyball.