The passive voice is a verb form that is used to describe actions or conditions. It changes the perspective of an action or event. While the active sentence emphasizes the subject carrying out an action, a passive construction places emphasis on the action carried out. The agent in a passive sentence is less important, perhaps not even named. The action itself is in the foreground. The subject of a passive sentence is usually the person or thing to which the process is done or happens.

Examples:
Active: Dr. Anderson operiert Harry.
Passive: Harry wird operiert.

Modal verbs can also be used to form passive constructions. They modify the meaning of the main verb just as they do in active sentences. "müssen" is an example of a modal verb. In active and passive sentences it expresses a necessity.

Examples:
Active: Wir müssen den Rasen mähen.
Passive: Der Rasen muss gemäht werden.
(= The passive sentence indicates that the lawn must be mown. There's no mention of who will do it because that doesn't matter or is not known.)

Forming the passive voice with modal verbs
To form the passive, German uses the helping verb "werden" and a past participle. To include a modal verb, it is conjugated and placed in the second position of an independent clause. The past participle goes to the end of the sentence, followed by the helping verb in its infinitive form.

As with all passive sentences, the accusative object from an active sentence becomes the subject and is placed at the beginning or directly after the finite verb. The agent (person or thing carrying out the action) can be included by adding the preposition "von".