Do the following sentences have a deontic or epistemic meaning?
- He must be studying now.
- He mustn't be studying now.
From the Linguistic modality entry on Wikipedia:
In English as in many other languages, some of the same words are used for deontic modality as for epistemic modality, and the meaning is distinguished from context: He must be there by now (epistemic) versus He must be there tomorrow at noon (deontic).
So each sentence could be either deontic or epistemic based on context.