In English language usage, “must” is typically not used to convey prohibition or something that is not allowed. Instead, “must” is commonly employed to express obligation or necessity.

When indicating prohibition or expressing something as not allowed, modal verbs like “cannot” or “must not” are more commonly utilized. “Cannot” explicitly states the impossibility or the prohibition of an action, while “must not” expresses a strong prohibition or a mandatory restriction on an action.

“Must not” is a more formal and forceful way of communicating prohibition compared to “cannot.” It conveys a sense of strict rules or regulations that must be followed.

In contexts where prohibition or forbidden actions are being emphasized, “must not” is preferred for its clarity and emphasis on compliance with rules or regulations. “Must not” is often used in official guidelines, rules, regulations, and warnings to clearly communicate what is prohibited or forbidden.

Therefore, while “must” conveys obligation or necessity, “cannot” and “must not” are more suitable for expressing prohibition or stating that something is not allowed in English language usage.