Uses of passive voice

Use of Passive
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.
Example: My bike was stolen.
In the exampleabove, the focus is on the fact that my bike was stolen. I do not know, however, who did it.
Sometimes a statement in passive is more polite than active voice, as the following example shows:Example: A mistake was made.
In this case, I focus on the fact that a mistake was made, but I do not blame anyone (e.g. You have made a mistake.).
Form of Passive
Subject + finite form of to be + PastParticiple (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 thesubject 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)Examples of Passive 
Tense | Subject | Verb | Object |
Simple Present | Active: | Rita | writes | a letter. |
| Passive: | A letter | is written | by Rita. |
Simple Past | Active: | Rita |wrote | a letter. |
| Passive: | A letter | was written | by Rita. |
Present Perfect | Active: | Rita | has written | a letter. |
| Passive: | A letter | has been written | by Rita. |
FutureI | Active: | Rita | will write | a letter. |
| Passive: | A letter | will be written | by Rita. |
Hilfsverben | Active: | Rita | can write | a letter. |
| Passive: | A letter | can be written| by Rita. |
Examples of Passive 
Tense | Subject | Verb | Object |
Present Progressive | Active: | Rita | is writing | a letter. |
| Passive: | A letter | is being written | by Rita. |Past Progressive | Active: | Rita | was writing | a letter. |
| Passive: | A letter | was being written | by Rita. |
Past Perfect | Active: | Rita | had written | a letter. |
| Passive: | A...