State the basis for an expectation
A timetable, calculation or observation can explain why a speaker expects something. Müsste does not make the conclusion infallible. In another context it can concern what someone ought to do. Look at the whole sentence and situation before deciding which meaning is intended.
Nach dem Fahrplan müsste der Bus bald hier sein.
According to the timetable, the bus should be here soon.
Optional does not mean forbidden
If an action is optional, deny its necessity with nicht müssen. If an action is prohibited, deny permission with nicht dürfen. Confusing these changes what the listener is allowed to do. A negative inference such as Das kann nicht stimmen. has yet another meaning: the speaker rules out the claim.
Du musst das Formular nicht ausdrucken.
You do not have to print the form.
Keep a possibility a possibility
A report that says a cause is possible has not established that cause. A faithful summary preserves this reservation. Likewise, an absence of proof does not prove that no connection exists. Explicit conditions and evidence often make writing more precise than a more elaborate modal expression.
Die Temperatur könnte eine Rolle spielen; belegt ist das bisher nicht.
The temperature could play a role; this has not been established so far.
Practise this pattern
The matching block includes guided questions, a separate mastery check and spaced review. The first 10 blocks and two samples at each higher level are free; other blocks require course access or an eligible seven-day trial. These tasks practise specific grammar patterns and do not certify a complete language level.
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References
Original explanations and examples informed by grammar references. Level placement is an editorial teaching choice.