Passive voice

The passive voice is made with the auxiliary verb to be and the past participle. The auxiliary verb shows the tense. The rubbish is collected every day. (present simple)
My suit is being cleaned. (present continuous)
He was invited to the party. (past simple)
The building was being renovated. (past continuous)
Two men have been arrested. (present perfect)
She had been interviewed earlier in the day. (past perfect)
Your request will be dealt with shortly. (will future)
Cars must not be parked here. (modal present)
It must have been stolen. (modal perfect)

*Note: perfect continuous tenses do not have a passive form.

It is important to understand the idea of a grammatical agent when studying the passive voice.
In English grammar, the agent is the person or thing that performs an action.
In an active (i.e. not passive) sentence, the agent is usually the subject. Jane wrote the letter.The subject Jane is also the agent. Now look at the passive:The letter was written by Jane. The subject is now the letter. Jane is not the subject, but she is still the agent.Sometimes, we do not need to say who performed an action. We may be interested in other information. For example, we might want to know when the letter was sent, but not who sent it.The letter was sent yesterday.We often use the passive voice when the agent is not important, we don't know who or what the agent is, or when we don't need to mention who the agent as it is already clear.

My bike has been stolen.
I don't know who stole my bike.The windows are cleaned once a week. We don't need to say the windows are cleaned once a week by the window cleaners. It is obvious that the window cleaners clean the windows. In this example, the agent is also unimportant as we are interested in when the windows are cleaned, not who cleans them.
Sometimes we make a passive sentence and want to mention the agent.
The new bridge will be opened by the transport minister on 1 May.
In this example, the subject is the bridge, perhaps because the sentence occurs in a text about the bridge, not the transport minister. (See note below.) However, we still want to mention the agent. This is an important public event, the agent is not obvious and people would want to know who will perform the ceremony.

Which of these two sentences is better?J K Rowling wrote the Harry Potter books.
The Harry Potter books were written by J K Rowling. The answer depends on which is more important in the context, J K Rowling or the Harry Potter Books. If you are writing a paragraph about J K Rowling, the first example is likely to be better; if you are focusing on the books, the second is probably a better choice.

Consider the example below: Three men broke into the National Museum last night. They stole over twenty famous paintings and escaped in a white van. Police officers caught them as they were driving to the airport. There are no grammar errors in this paragraph, but the style is awkward. This is because the subject changes in the last sentence from three men to police officers. We prefer to continue with the same subject where possible: Three men broke into the National Museum last night. They stole over twenty famous paintings and escaped in a white van. They were caught by police officers as they were driving to the airport.

Compare the following pairs of examples: The electric light bulb has changed the lives of millions. Edison invented it in the late 19th century.
The electric light bulb has changed the lives of millions. It was invented by Edison in the late 19th century. The grapes are picked and put into plastic tubs. Then we take them to the winery and press them.
The grapes are picked and put into plastic tubs. Then they are taken to the winery and pressed. In both cases, the second example sounds better as the same subject is used for the second sentence.
Compare the following:
The company has announced its latest smartphone, the Galactic 8. They will release the new model next month.
The company has announced its latest smartphone, the Galactic 8. The new model will be released next month.
The second example is better. The focus changes to the Galactic 8, so this previously mentioned information (or a synonym - The new model) is put at the beginning of the next sentence, which gives more details about it.

You must settle your bill before departure. You must pay this bill by 31 July, otherwise we will cut off your electricity supply.There are no grammar errors in these sentences, but they appear very direct and rude. In these examples the passive would be a better choice because it is less personal.
All bills must be settled before departure.This bill must be paid by 31 July, otherwise the electricity supply will be disconnected. A more personal tone is sometimes preferable. Compare the following: I will follow up your inquiry and let you know the outcome as soon as possible.
Your inquiry will be dealt with and you will be informed of the outcome as soon as possible. The first example is more personal and thus more reassuring than the second. Very often, the choice depends on who is writing. If the writer is communicating personally with the recipient, the passive voice can sound cold and unhelpful.
Examination results will be sent out on 1 November.
Bags must not be left here.
Expenses claims must be submitted by the end of the month.