sollen or müssen?
a2Both express an obligation, but they differ in where it comes from: müssen arises from circumstances or compulsion, sollen comes from another person who wants or says so.
müssen
Compulsion from the matter itself: a law, a necessity, a fact of nature. Nobody has to tell you.
Ich muss zum Arzt, mein Bein tut weh.I have to see a doctor, my leg hurts.
sollen
Another person’s will: an instruction, a piece of advice, an expectation. The source is often named: Der Arzt sagt, ich soll …
Der Arzt sagt, ich soll mehr schlafen.The doctor says I should sleep more.
The mistake
Meine Chefin sagt, ich muss am Samstag arbeiten - gemeint ist ihre Anweisung, nicht ein Zwang.
Meine Chefin sagt, ich soll am Samstag arbeiten.
If the person the obligation comes from is already in the sentence, sollen belongs there. Müssen would make the boss redundant: it would have to happen anyway.
Rule of thumb
Ask who wants it. If you answer with a name, take sollen. If you answer with the circumstances, take müssen.
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