A second job for modal verbs
Er muss arbeiten means he has to work. But Er muss krank sein does not put him under any obligation to be ill. The modal now describes your own judgement: you see the evidence and conclude he is ill. Two modals, sollen and wollen, name the source of a claim instead.
Your confidence: muss, dürfte, könnte, kann nicht
Four forms grade how far you commit, from near-certainty down to a possibility. kann nicht is the opposite end: ruled out. dürfte is the one to watch: it means probably, not would be allowed to.
Das muss ein Missverständnis sein.
That must be a misunderstanding.
müssen: as good as certain
Der Zug dürfte gleich kommen.
The train should be here any minute.
dürfte: very likely
Sie könnte noch im Büro sein.
She might still be at the office.
könnte: possible, no more
Das kann nicht dein Ernst sein!
You can't be serious!
kann nicht: ruled out
To say something is impossible, use kann nicht. Er muss nicht krank sein only says he is not necessarily ill.
Other people's claims: sollen and wollen
Both are very common in news and gossip, whenever you pass something on without vouching for it.
sollen: somebody else says so
Third parties say it, and you stay neutral: it is said that, supposedly. In speech this is the everyday shortcut for reported speech.
Das neue Café soll richtig gut sein.
The new café is supposed to be really good.
Es soll morgen den ganzen Tag regnen.
It's supposed to rain all day tomorrow.
wollen: the subject says so about himself
It is the subject's own claim, with audible doubt: he claims to. You would not use it about something you believe.
Sie will den Unfall nicht gesehen haben.
She claims not to have seen the accident.
Er will das ganz allein erfunden haben.
He claims to have invented it all by himself.
Guessing about the past
The modal stays in the present; the infinitive goes into the past. English does the same (he must have forgotten).
Er muss krank gewesen sein.
He must have been ill.
Sie dürfte den Bus verpasst haben.
She has probably missed the bus.
Der Brand soll in der Küche ausgebrochen sein.
The fire is said to have started in the kitchen.
Compare the objective past, where the modal itself goes into the Perfekt and becomes an infinitive at the end: Er hat gestern lange arbeiten müssen (he had to work late) against Er muss gestern lange gearbeitet haben (he must have worked late).
