Bücher müssen geprüft werden.

Books must be checked.

Breakdown of Bücher müssen geprüft werden.

• das Buch: the book
• müssen: must
• geprüft werden: to be checked

Questions & Answers about Bücher müssen geprüft werden.

What voice is used in Bücher müssen geprüft werden?

This is the German passive voice (Vorgangspassiv), which focuses on the action of checking (“prüfen”) rather than who does it.

Why is Bücher in the nominative case even though it’s the object in the English translation “Books must be checked”?

In German passive sentences, the original direct object (what would be in the accusative in an active sentence) becomes the grammatical subject and thus takes the nominative case. Here, Bücher was the object of an active sentence “(Jemand) prüft Bücher,” so in the passive it becomes the subject.

Why are there two verb forms at the end—geprüft werden—instead of a single verb?

Because you have both a passive construction and a modal verb (müssen). With a modal in German passive, you use a double‑infinitive:

• geprüft is the past participle of prüfen (to check)
• werden is the infinitive used to form the passive
  Both go to the end after the modal.

Why does müssen appear in the second position of the sentence?

German main clauses follow the V2 (Verb‑second) rule: the finite verb always occupies the second position. Here the first position is taken by the subject Bücher, and then you place the conjugated modal müssen.

How would you add an agent—“by someone”—to this passive sentence?

Use von plus the agent in the dative:
• Bücher müssen von den Lehrern geprüft werden.
(Books must be checked by the teachers.)

Can I change the tense of this sentence? For example, to past or perfect?

Yes. For Präteritum (simple past):
• Bücher mussten geprüft werden.
For Perfekt (present perfect) with a passive + modal:
• Bücher haben geprüft werden müssen.
(It’s a bit complex: you keep the double infinitive geprüft werden müssen at the end and use haben as the auxiliary.)

Why is there no article before Bücher?

Omitting the article makes it a general statement about all books. If you refer to specific books, you’d include an article or demonstrative:
• Die Bücher müssen geprüft werden. (The books must be checked.)

What’s the difference between this passive and a “lassen”‑passive like Bücher lassen prüfen?

• lassen‑passive (Zustandspassiv) often implies arranging or causing something to happen, e.g. making sure someone checks the books.
• werden‑passive (Vorgangspassiv) describes the process itself: the books must undergo checking.