Решение:
- When we entered the classroom, we were having lunch when our friends called us.
- Before they had gone to the swimming pool, they (eat) breakfast.
- Yesterday Ben felt happy because she (wake up).
- Sarah had studied for the test for two hours, so he (break) the window.
- By the end of last year, I had learned a lot of new English words (meet) her teacher, she (win) the competition.
- When Alice learned her keys at school, she (find) out that she (build) the table, he put it in the kitchen.
Ответ: 1. were having; 2. had gone; 3. felt; 4. had studied; 5. had learned; 6. left.