You already know how the passive voice (Passiv) is built with the auxiliary werden, and how modal verbs work on their own. At this level you learn how these two structures combine into a single construction expressing obligation, possibility, or permission in a sentence with no personal subject — the dominant style of formal German. Laws, instructions, forms, and official announcements are almost all written this way, because it removes the impersonal subject man from the sentence and puts the entire focus on the thing that needs to be done.
Grammatically, the modal verb remains the one that is conjugated for subject and tense and that occupies the second position in the main clause, exactly as usual. But instead of being followed directly by the main verb's infinitive, as in the active voice, it is followed by a two-part construction pushed to the very end of the sentence: the Partizip II of the main verb, followed by werden in its infinitive form (not conjugated). The active sentence Man muss das Formular ausfüllen becomes Das Formular muss ausgefüllt werden, where the object das Formular becomes the grammatical subject, while ausgefüllt and werden move together to the end of the clause.
In other tenses, only the modal verb's conjugation changes — for example, in the Präteritum, Das Formular musste ausgefüllt werden — while the final ausgefüllt werden stays fixed across almost every tense, since tense is carried entirely by the modal verb.
There are also three shorter alternatives that convey the same meaning without an explicit Passiv, all very common in official texts: sein + zu + infinitive, where Das Formular ist auszufüllen means exactly the same as muss ausgefüllt werden; sich lassen + infinitive, which expresses possibility, so Das lässt sich machen means kann gemacht werden; and adjectives derived with the suffix -bar, which convey capability, such as lesbar, "readable," meaning "can be read."
The most common mistake is using sein instead of werden in this construction, confusing it with the Zustandspassiv, which describes a resulting state rather than an action. Another frequent error is forgetting to push the Partizip II and werden together to the end of the sentence, leaving them instead in the position where the main verb would go in an active sentence.