Ответ:
1. At twelve o'clock the workers were loading the trucks.
At twelve o'clock the rucks were being loaded (by the workers).
2. By three o'clock the workers had loaded the trucks.
By three o'clock the trucks had been loaded (by the workers).
3. We send our daughter to rest in the south every year.
Our daughter is sent to rest in the south every year.
4. They will show this film on TV.
This film will be shown on TV.
5. They are building a new concert-hall in our street.
A new concert-hall is being built in our street.
6. They have made a number of important experiments in this laboratory.
A number of important experiments have been made in this laboratory.
7. Livingstone explored Central Africa in the 19th century.
Central Africa was explored by Livingstone in the 19th century.
8. By the middle of autumn we had planted all the trees.
All the trees had been planted by the middle of autumn.
9. They will stage this play at the beginning of next season.
This play will be staged at the beginning of next season.
10. They have forgotten the story.
The story has been forgotten.
11. Has anybody explained the rules of the game to you?
Have you been explained the rules of the game?/ Have the rules of the game been explained to you?
12. They haven't brought back my skates.
My skates haven't been brought back.
Объяснение: