The negative form of the modal verb “must” in English is “must not” or “mustn’t.” When “must not” is used to express prohibition or impossibility, it signifies that something is not allowed or should not happen. It conveys a sense of obligation to refrain from doing a specific action.

Comparing “must not” to other negative modal verbs like “cannot” or “may not” in terms of expressing prohibition or impossibility, each of these modal verbs carries a slightly different connotation:

– “Cannot” implies the inability to do something or a lack of capability, often focusing on external factors preventing an action.

– “May not” suggests a possibility being restricted or disallowed, typically related to permission or likelihood.

In contrast, “must not” emphasizes a strong necessity or obligation to avoid a particular action, with a sense of duty or moral imperative attached. It indicates a clear prohibition or rule that must be followed.

Therefore, when using “must not” to express prohibition or impossibility in English, it conveys a firm restriction or directive that must be adhered to, distinguishing it from the nuances conveyed by “cannot” or “may not” in similar contexts.