Ответ:

  1. When I came home, my mother was cooking dinner. Краткое событие произошло на фоне длительного действия.
  2. They were playing chess while I was reading a magazine. Два действия происходили одновременно.
  3. She broke her leg last winter. Указан завершённый период в прошлом.
  4. What were you doing at 8 o’clock yesterday? Спрашивается о действии в конкретный момент в прошлом.
  5. He was walking down the street when he saw an old friend. Краткое событие произошло во время длительного действия.
  6. We did not watch TV yesterday.
  7. The sun was shining and the birds were singing when we left the house. Оба действия происходили в тот момент.
  8. Did you meet him at the party last week?