To express a strict prohibition ("must not"), use the negative modal verb ne smjeti.
Crucially, ne smjeti strictly requires the imperfective aspect for the infinitive verb that follows it. Grammar-wise, you are forbidden from even engaging in the ongoing process of the action, even for quick events like stepping into a zone.
| Nobody is allowed to open the confidential archive. | Nitko ne smije otvarati povjerljivu arhivu. |
| We must not enter the red zone without an escort. | Ne smijemo ulaziti u crvenu zonu bez pratnje. |
Lack of Obligation: Ne morati
If you simply "do not have to" do something, use ne morati. This expresses a lack of obligation rather than a ban.
Unlike ne smjeti, ne morati gives you aspectual freedom. You can easily follow it with a perfective verb for a single, completed action.
| You don't have to read the document, although it is confidential. | Ne morate pročitati dokument, iako je povjerljiv. |
Contrasting the Modals
When combining both modals in a single sentence, the aspectual rules remain tied to their respective verbs. The action you don't have to do can be perfective (like entering or hiding your identity), but the action you must not do stays strictly imperfective (like scanning or recording).
| You do not have to enter the zone today, but you must not scan anything. | Ne morate ući u zonu danas, ali ne smijete ništa skenirati. |
| We do not have to hide our identity, but we must not record anything in the archive. | Ne moramo sakriti svoj identitet, ali ne smijemo ništa snimati u arhivi. |