Deontic modality is a linguistic modality that indicates how the world ought to be, according to certain norms, expectations, speaker desire, etc. In other words, a deontic expression indicates that the state of the world (where 'world' is loosely defined here in terms of the surrounding circumstances) does not meet some standard or ideal, whether that standard be social (such as laws), personal (desires), etc. The sentence containing the deontic modal generally indicates some action that would change the world so that it becomes closer to the standard or ideal.
This category includes the following subcategories:
Commissive modality (the speaker's commitment to do something, like a promise or threat): "I shall help you."
Directive modality (commands, requests, etc.): "Come!", "Let's go!", "You've got to taste this curry!"
Volitive modality (wishes, desires, etc.): "If only I were rich!"
A related type of modality is dynamic modality, which indicates a subject's internal capabilities or willingness as opposed to external factors such as permission or orders given.
