Passive with must and can
With a modal verb, put Partizip II plus werden at the end. The modal agrees with the subject. In a subordinate clause the finite modal normally comes last.
Understand the idea
A modal can state what must, may or can happen to something without naming its actor. The finite modal agrees with the passive subject. The lexical verb appears as a participle, followed by the passive infinitive werden.
Build it step by step
In a main clause, place the finite modal in the usual position and put participle plus werden at the end. In a basic subordinate clause, the finite modal follows that passive combination. Keep the participle independent of the subject's person and number; agreement appears on the modal.
See it in a sentence
Die Rechnung muss heute bezahlt werden.
Das Fenster kann geöffnet werden.
A useful memory aid
Assemble the ending first: repaired + werden. Then add a modal meaning and conjugate that modal for the subject.
A common trap
Do not replace the participle by an ordinary infinitive. A passive modal construction needs the lexical participle as well as werden.