Нужно задать вопрос к каждому члену предложения.
1. I was working in our lab at 10 yesterday
2. He was preparing for the English test when his sister come.
3 they were discussing the results of their test from 11 or 12 yesterday.
4. We were playing football on the sports-ground at 8 o clock
5. Mary was reading a letter from her friend when her mother came