Modal Verbs (können, müssen, sollen, wollen, dürfen, mögen)
What's the difference between "sollen" and "müssen"?
müssen: Strong obligation, internal necessity, or external unavoidable duty ("must," "have to").
sollen: Weaker obligation, external command/suggestion, rumour, or moral duty ("should," "ought to," "am supposed to").
Example
Ich muss arbeiten, um Geld zu verdienen. (I have to work - internal necessity)
Du sollst deine Hausaufgaben machen. (You should do your homework - external expectation/advice)