How to identify whether the sentence …

1. “By” is used in the passive voice when the actor needs to know the job.
   Example: “Love addicted” was sung by Vamps.
2. Passive voice is used if it doesn’t need to know the perpetrator work.
   Example: the streets are cleaned everyday
3. Passive voice is used if we don’t know or forget who the perpetrator work.
   Example: The police was murdered.
4. Passive voice is used if we are more interested in the job than the actors who work.
   Example: A new departmental store is being built.
5. Passive voice is used to avoid an awkward sentence or inappropriate with grammar.
   Example: When she arrived home a police arrested her — it’s better: when she arrived home she was arrested (by a police).

The places of subject and object are interchanged i.e. the object shifts to the place of subject and subject shifts to the place of object in passive voice.

Example:

• Active voice: I write a letter.
• Passive voice: I letter is written by me.

Subject (I) of sentence shifted to the place of object (letter) and object (letter) shifted to the place of subject (I) in passive voice.