Chapter I. General information about grammatical categories of verb in english grammar 6

Mood and Modality
A distinction should be made between grammatical mood and semantic
modality.⁴ Mood is a matter of grammatical form, modality a matter of meaning.
The main markers of modality in English are the modal auxiliaries can, may, must,
will, shall, together with a few less central ones. When considering modality it is useful to distinguish between two parts: the dictum: what is said
the modus: how it is said (that is, the speaker's cognitive, emotive, and/or
volitive attitude about what is said).
For example, a sentence could have the following dictum: It is hot outside.
This dictum could be paired with various of modi, such as the following:

I hope that it is hot outside.
I doubt that it is hot outside.
It must be hot outside.
It has to be hot outside.
It might be hot outside.
It could be hot outside.
It needn't be hot outside.
It shouldn't be hot outside.
It is probably hot outside.
Perhaps it is hot outside.
It is possible that it is hot outside.
It is certain that it is hot outside.
It is probable that it is hot outside.
It is likely that it is hot outside.

Three main kinds of modal meaning are distinguished:

1. 
   deontic,
   
2. 
   epistemic,
   
3. 
   dynamic.
   

Deontic modality is concerned with “influencing actions, states, or events”
and typically has to do with such notions as obligation and permission, or – in
combination with negation – prohibition. Deontic modal meaning also deals with
threats, promises (commissive deontic modality), requests, commands, instructions
(directive), desires, wishes and fears (volitive):
e.g. You must come in immediately (obligation).
You can have one more turn. (permission)
You can’t have any more. (prohibition)
May he lose the race. (wish)
Epistemic modality is concerned with the speaker’s judgement of the truth
of the proposition embedded in the statement. e.g. It was a mistake represents an unqualified assertion. It must have been a mistake suggests that I am drawing a conclusion from evidence rather than asserting something of whose truth I have direct knowledge.
You may be right merely acknowledges the possibility that “You are right” is
true. Dynamic modality has nothing to do with the speaker, it is subject-oriented
and generally concerns the properties and dispositions of persons, etc., referred to
in the clause. e.g. Liz can drive better than you. I asked Ed to go but he won’t.
In these examples the speaker is concerned with Liz’s driving ability and
Ed’s willingness to go. All three kinds of modality are commonly expressed by other means than by modal auxiliaries: lexical verbs (You are obliged to do that), adjectives (You are likely to be fined), adverbs (Perhaps you are right), nouns (You have my permission to leave early).⁵