Once you've learned the basic passive with werden, you'll often need to combine it with modal verbs like müssen, können, and dürfen, to say things like "the form must be filled out" or "the problem can be solved" without mentioning who does it. This construction, Passiv mit Modalverben, is extremely common in official instructions, contracts, and the workplace, because it packs both the necessity or possibility of an action — the modal verb — and the neutrality about who performs it — the Passiv — into one compact sentence.
The core idea is simple once you know the two rules behind it: how Passiv is built with werden + Partizip II, and how modal verbs are conjugated and push the main verb to the end. Here you combine the two: the modal verb is conjugated in the normal second position, while everything else — Partizip II followed by werden in its infinitive form — moves to the very end. werden always stays plain and unconjugated here, since it carries the passive meaning, while the modal verb is the one conjugated up front.
For example, the active sentence "Man muss das Formular ausfüllen" (one must fill out the form) becomes "Das Formular muss ausgefüllt werden": the vague subject "man" disappears, Formular becomes the subject, muss stays in second position, and ausgefüllt werden move together to the end.
Things get slightly more complex in subordinate clauses starting with dass or weil, because the rule sending the conjugated verb to the very end also applies to the modal verb, giving the order Partizip II, then werden, then the conjugated modal verb as the last word: "Ich weiß, dass das Formular ausgefüllt werden muss."
This construction often has no neat one-to-one equivalent elsewhere, since many ways of phrasing the same idea use a plain sentence like "the form must be filled out" without changing the verb's form at all — German instead needs three elements in a strict, non-negotiable order.
The most common mistake is forgetting werden at the end and stopping at the modal verb plus Partizip II alone, producing the incorrect "Das Formular muss ausgefüllt" instead of the complete "Das Formular muss ausgefüllt werden." werden never disappears here — it's what makes the sentence passive in the first place.