1.When can we finish the work?
When can the work be finished (by us)?
2.A lot of people in the world use English.
English is used by a lot of people in the world.
3.The young man saved the lives of the children.
The lives of the children were saved by the young man.
4.His parents will buy him a new bike.
He will be bought a new bike by his parents.
A new bike will be bought for him by his parents.
5.She is singing a new song.
A new song is being sung by her.