In passive sentences, the thing receiving the action is the subject of the sentence and the person or thing doing the action is optionally included near the end of the sentence with the preposition by added before the doer.
We can use the passive form if we think that the thing receiving the action is more important or should be emphasised. And we can also use the passive form if we do not know who is doing the action or if we do not want to mention who is doing the action.
General form of the Passive Voice is:
Thing/person receiving action + be + 3rd form of the verb + other parts of the sentence.
Present Simple Passive:
Thing/person receiving action + am/is/are + 3rd form of the verb + other parts of the sentence.
These souvenirs are made in China.
This book is often read (by many people).
Past Simple Passive:
Thing/person receiving action + was/were + 3rd form of the verb + other parts of the sentence.
This book was read yesterday.
The work was finished yesterday.
Present Perfect Passive:
Thing/person receiving action + have/has + been + 3rd form of the verb + other parts of the sentence.
The book has already been read.
The letter has been written.