i do not understand

Ответ от Olga Tishineva[гуру]
1. The letter WAS WRITTEN some days ago.
2. This plant WAS RECONSTRUCTED last year.
3. These cars ARE PRODUCED by one of the plants in our city.
4. All these cables WILL BE ANSWERED tomorrow.
5. While he was having lunch, a message WAS BROUGHT to him.
6. The problem WAS BEING DISCUSSED when the telephone rang.
7. We soon became good friends, and very often when the work HAD BEEN DONE we went out together.
8. The new desk WAS PLACED at the window, and he could see the street, trees and flower-beds.
9. Progress in technology today IS ASSOCIATED with the peaceful use of atomic energy.
PASSIVE VOICE TO BE + VIII
(глагол to be - вспомогательный, все изменения происходят с ним, смысловой глагол в третьей форме остаётся без изменения)
Сводную таблицу страдательного залога можно посмотреть вот здесь