Passive voice
Definition
Voice is one of the categories of the verb. We differentiate between active and passive voice. Passive voice can be used only in sentences which contain transitive verb. In case that the sentence has more than one object, every one of them can become the subject of the passive structure. When changing the active sentence into the passive one, the active subject becomes the passive agent, the active object becomes the passive subject and the preposition by can be optionally added to introduce the real agent of the action.

Usage
Passive voice is used in these situations:

1.when the agent of the action in unknown, unimportant or generally obvious,

“My office was broken into when I was on holiday.”(unknown)
“These boxes should be handled with care.”(unimportant)
“She is being treated in hospital.”(obvious)

2.in academic, factual or technical writing to omit the agent,
“Nuclear waste can be stored as a liquid in stainless-steel containers which are encased in concrete.”

3.to avoid responsibility,
“It has been decided to reduce all salaries by 10 %.”

4.to emphasise the agent,
“Jack won the prize.” - “The prize was won by Jack.”

5.to put long subject expressions at the end of a sentence,
“I was surprised by Don’s decision to give up his job and move to Sydney.”

6.to put old information at the beginning of a sentence or clause,
“The three machines tested for the report contained different types of safety valve. The machines were manufactured by the Boron Group in Germany.”

7.to report some piece of information.
“It has been acknowledged that underfunding is part of the problem.”

Using modal auxiliary verbs with passive voice