The passive voice is less common than the active voice but has its uses. It is especially helpful in scientific writing where the actor is unimportant and the process being described is key. The passive voice focuses on the action rather than who or what is performing the action. When changing an active sentence to the passive voice, the object of the active sentence becomes the subject of the passive sentence and the subject of the active sentence may become the object or be dropped from the passive sentence.

4. In the active voice, the subject and verb
relationship is straightforward: the
subject is a be-er or a do-er and the
verb moves the sentence along.
Active voice is a grammatical
voice common in many of the world's
languages. It is the unmarked voice for
clauses featuring a transitive
verb in nominative–accusative
languages.

8. The passive voice is less usual
than the active voice. While the
Passive Voice is the “normal”.
Computerized grammar checkers
can pick out a passive voice
construction from miles away and
ask you to revise it to a more
active construction,

9.  The passive voice is especially helpful (and
even regarded as mandatory) in scientific or
technical writing or lab reports, where the actor
is not really important but the process or
principle being described is of ultimate
importance. Instead of writing "I poured 20 cc of
acid into the beaker," we would write "Twenty cc
of acid is/was poured into the beaker." The
passive voice is also useful when describing,
say, a mechanical process in which the details
of process are much more important than
anyone's taking responsibility for the action:
"The first coat of primer paint is
applied immediately after the acid rinse."

10. Passive voice is used when
the focus is on the action. It is
not important or not known,
however, who or what is
performing the action.

11. Subject + finite form of to be + Past Participle (3rd
column of irregular verbs)
 Example: A letter was written.
 When rewriting active sentences in passive voice,
note the following:
 the object of the active sentence becomes the
subject of the passive sentence
 the finite form of the verb is changed (to be + past
participle)
 the subject of the active sentence becomes the
object of the passive sentence (or is dropped)

16.  http://grammar.ccc.commnet.edu/GRAMMAr
/passive.htm
 https://webapps.towson.edu/ows/activepass.
htm