There is a difference between the two types of use of modal verbs. If we use an epistemic modal, we are interested in the truth of a proposition. If we use a deontic modal, we are interested in the occurence of an event.
Epistemic modals could be interpreted as subjectively epistemic or
objectivelyepistemic. The difference between the two systems is pragmatic. When a sentence is subjectively modalized, it means that the speaker wants to show hislher attitude towards the proposition. Thus, the sentence includes an expression of an opinion. An objectively modalized sentence is based on an evidence or fact that the speaker's opinion is based on. In this case, the sentence includes a factual statement.
There are some restrictions on the use of modal verbs in the two systems. A modal verb with subjectively epistemic meaning cannot be put in a negative form.
When a sentence with a modal verb with subjectively epistemic meaning does contain a negation, the negation is morphological. Modal verbs with objectively epistemic meaning can be negated. Then it is a negation ofthe sentence.
