Verb forms:
 Present simple am/are/is + past participle
Past simple was/were + past participle
Present continuous am/are/is + being + participle
Present continuous am/are/is + being + participle
 Past continuous was/were + being +participle
 Present perfect have/has been + participle
Past perfect had been + participle
Future simple will be+participle
Future simple will be+participle
Going to am/are/is + going to be + participle
Modal verb modal verb + be + participle
Infinitive (to) be + pp OR (to) have been + participle
| 
He writes a book | 
A book is written | 
| 
He is writing a book | 
A book is being
  written | 
| 
He wrote a book | 
A book was written | 
| 
He was writing a
  book | 
A book was being
  written | 
| 
He has written a
  book | 
A book has been
  written | 
| 
He had written a
  book | 
A book had been
  written | 
| 
He will write a book | 
A book will be
  written | 
We use the passive voice when who or what causes the action is unknown or unimportant or obvious or 'people in general':
- He was arrested (obvious agent, the police).
- My bike has been stolen (unknown agent).
- The road is being repaired (unimportant agent).
- The form can be obtained from the post office (people in general).
In  factual or scientific writing:
- The chemical is placed in a test tube and the data entered into the computer.
Here's a list of exercises for practice with the passive
 
