Passive Voice

To change a sentence from the active to the passive, change three things:

1. accusative object of active sentence to nominative subject of passive sentence
2. active verb to a tense of werden plus the past participle of verb in active sentence
3. subject to von + dative object in the passive sentence, if agent is mentioned

Present Tense
Viele Studenten lesen diesen Roman. = Dieser Roman wird von vielen Studenten gelesen.
Many students read this novel. = This novel is read by many students.

Imperfect Tense
Viele Studenten lasen diesen Roman. = Dieser Roman wurde von vielen Studenten gelesen.
Many students read this novel. = This novel was read by many students.

Future Tense
Viele Studenten werden diesen Roman lesen. = Dieser Roman wird von vielen Studenten gelesen werden.
Many students will read this novel. = This novel will be read by many students.

Present Perfect Tense
Viele Studenten haben diesen Roman gelesen. = Dieser Roman ist von vielen Studenten gelesen worden.
Many students have read this novel. = This novel has been read by many students.

Past Perfect Tense
Viele Studenten hatten diesen Roman gelesen. = Dieser Roman war von vielen Studenten gelesen worden.
Many students had read this novel. = This novel had been read by many students.

*Notice that in the passive voice, the past participle of werden is worden and not geworden.

Durch can replace von when the agent is an impersonal force (fire, wind, etc.); but it cannot be used if preceded by a limiting word (such as an article or adjective.)

Passive with modals
Shifts in tense will only affect the modal part of the sentence.  The infinitive forms of the past participles are used with modals in the passive voice as well.  And where you might expect something like Das Haus hat werden müssen verkauft, the actual construction is Das Haus hat verkauft werden müssen because of the double infinitive construction.  Double infinitives always go to the end of the sentence, but you only need to worry about these in the present perfect and past perfect tenses.

Problems with the Passive

False Passive

Grammatically, the false passive is the same as sein + an adjective.  This construction describes a condition rather than an action.  Das Haus ist verkauft is the false passive, while das Haus wird verkauft is the true passive.  The false passive sentence indicates that the house is already sold (condition), while the true passive indicates the house is in the process of being sold (action).

Passive with Absentee Subjects
Passive forms may have a definite or indefinite subject, or no apparent subject at all.  The accusative object of an active sentence becomes the nominative subject of the passive sentence.  But sometimes there is no accusative object.  Since a verb cannot be in the first position of sentence without turning the sentence into a question, es is used as the subject.  

Man antwortet ihnen nicht is an active sentence, but if it were turned into the passive, there would be no accusative object.  The passive would have to be es wird ihnen nicht geantwortet.  (Here werden agrees with the apparent subject, es.)

But if another element, such as a dative object or time expression, can be put in the first position, then es is omitted.  Ihnen wird nicht geantwortet can also be used as the passive.  There is no apparent subject, only an implied es, so the form of werden remains wird to agree with es.