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

1. When was you working in our lab? Where was you working? Who was working? 2.Who was prepairing for the English test?When was he preparing for the English test? When did his sister come? What did his sister do?3. Who were discussing the result of their test? What were they discussing? When were they discussing the result?4. Who were playing football? what were we playing? What were we doing? Where were we playing football? When were we playing football?5. Who was reading a letter? What was Mary reading? Who was write Mary letter? When did come her mother?