The email was sent, the report written, the contract signed today.
Who did those things? We don't know because those are passive sentences, we seem to care more about the actions involved! When those actions involve a modal verb (should, can, must), then nothing really gets too complicated.
The email should be sent, the report must be written, the contract can be signed today.
German is quite similar here, remember we don't use the auxiliary "to be" but werden instead.
Die E-mail soll heute geschickt werden.
Der Bericht muss heute geschrieben werden.
Der Vertrag kann heute unterschrieben werden.
In German, all infinitives go in the end of the sentence, "werden" here is the infinitive. Meanwhile, the main verb is the modal verb. How do you know? Because it's the only conjugated verb (also called finite verb or the one with the funny endings). Finite verbs go in second position right after the subject. What?? Passive sentences do have a subject after all? Yes, the subject of every passive sentence is what in the active sentence is the direct object. So there is an inversion:
I write the report. (direct object)
The report is written (by me). (subject)
And that leaves us with the participle ("ge-verb") which goes just before the infinitive and should never ever be separated from its buddy the auxiliary "werden".
Subject + Finite verb + all other information + participle + "werden" (Eine E-Mail muss heute noch geschrieben werden) = a great passive sentence with a modal verb!