While in the active voice the action is performed by the person who is the subject of the sentence, in the passive voice it is performed by somebody else. If the doer of the action is indicated, it is denoted by an object with the preposition by.
All these documents were signed by the general manager.
Все эти документы были подписаны управляющим.
The Passive Voice is used in the following cases:
When the doer of the action is unknown.
The doctor said that he had been killed two hours ago.
Врач сказал, что он был убит два часа назад.
When the doer of the action is known but it is less important than the object of the action.
This poem was written by Mayakovski in 1924.
Эта поэма была написана Маяковским в 1924.
To form the Passive Voice one should take the auxiliary verb to be in the correspondent tense and add Participle II of the notional verb.
Note: The tenses of the Present Continuous group in Passive Voice are replaced by the correspondent tense of the Perfect group.
The question is being discussed.
This question has been discussed.
Этот вопрос обсуждают.
This question has been discussed since the very morning.
Этот вопрос обсуждают с самого утра.
The Passive Voice is much more common in English than in Russian and there is a difference between the use of the Passive Voice in English and in Russian.
Мой шеф дал мне эти материалы только вчера.
My boss gave me these materials yesterday only.
Эти материалы были даны мне только вчера.
These materials were given to me yesterday only.
I was given these materials yesterday only.
While in Russian it's only a direct object which can became the subject of the Passive Voice, in English both a direct object and an indirect one, even an indirect prepositional object can become the subject of the correspondent sentence in the Passive Voice, in latter case the preposition is placed to the end of the sentence after the notional verb.
These data are often referred to.
На эти данный часто ссылаются.
Your plan has been approved of.
Твой план был утвержден.
Note: Sometimes even an adverbial modifier of place can become the subject of the Passive Voice.
This house is not lived in.
В этом доме никто не живет.
The police found that the bed had not been slept in.
Полиция обнаружила, что в этой кровати не спали.
Verbs of speaking in Passive Voice.
He was said to be very rich. - Говорили, что он очень богат.
He was told that she had left. - Ему сказали.
He was told to go and buy some bread. - Ему велели.
He was spoken (talked):
He was much spoken (talked) about
О нем много говорили.
He was spoken (talked) about (to) it yesterday.
С ним вчера говорили об этом.