Помогите пожалуйста!
Объясните пожалуйста, отмечу как лучший ответ.
1 . I ( to go ) to the theatre yesterday . 2 . At seven o'clock yesterday I ( to go ) to the theatre . 3 . What you ( to do ) at 5 o'clock yesterday ? - I ( to play ) the piano . 4 . When I ( to come ) to school , the children ( to stand ) near the classroom . 5 . We ( to play ) in the yard the whole evening yesterday . 6 . When I ( to prepare ) breakfast in the morning , I ( to cut ) my finger . 7 . Last year I ( to go ) to the United States . 8 . You ( to go ) to Great Britain last year  ? - No , I ( to go ) to France . 9 . What you ( to do ) yesterday ? - I ( to translate ) a very long article . 10 . When I ( to ring ) up my friend , he ( to sleep ) .
Вот, ответы. 
1. I went. 2. I was going. 3. What were you doing, I was playing. 4. I came, the children were standing. 5. We were playing. 6. I was preparing, I cut. 7. I went. 8. Did you go, I went. 9. What did you do, I translated. 10. I rang, he was sleeping.
Мне надо только объяснение, пожалуйста!