Unknown Subjects

In the following sentences, if the passive is used, the "by" phrase will probably be deleted because the doer of the action is clearly understood.

The construction workers finished the building on time.
The building was finished on time. (in general, only construction workers build buildings)

Fireman put out the fire and saved the little child.
The fire was put out and the little child was saved. (in general, firemen put out fires)

In these sentences, if the passive is used, the "by" phrase is retained to identify the doer of the action.

Mr. Blandings built a deck for his house.
A deck for his house was built by Mr. Blandings. (it wasn't built by construction workers)

A quick-thinking young boy put out the grass fire.
The grass fire was put out by a quick-thinking young boy. (it wasn't put out by firemen)