Passive sentences with "by"

We are normally not interested in the "doer" of an action in a passive sentence. When we want to mention the "doer", we use the preposition by. The whole phrase is called by-agent in English. 

Active sentence	Passive sentence
Mr Brown built the house.	The house was built by Mr Brown.

When we do not know, who was the "doer" of the action, we use someone or somebody in the active sentence. We leave out these words in the passive sentence.

Active sentence	Passive sentence
Someone stole my bike.	My bike was stolen.

Related Posts :

Active and Passive Voice

• Use-Passive and Active, Form and Examples
• Active and passive forms in the most common tenses