Typically, modal expressions which express degrees of likelihood can occur only where assertion, which is the claim of truth or untruth of a proposition, is possible. This is because nlodal expressions cannot designate a degree of likelihood where no such degree is at stake. This observation has several consequences. First, modal expressions that express higher degree of probabmty cannot occur in normal yes-no questions:
(1) Has John {*surely / *certainly / ? probably / possibly / perhaps} gone to the movie ?
(2) {*Must/*May/Might} he be reading a book now ?
Secondly, they cannot normally occur in if-clauses, in which truth value of a proposition is irrelevant:
(3) *If the report is {surely / certainly / probably / possibly perhaps} true, we must be prepared for the worst.
(4) *If the report {must / may / might} be true, we must be prepared for the worst.
Thirdly, they do not usually occur in the scope of negation:
(5) a. The report is surely not true.
b. *The report is not surely true.
(6) John may not be reading a book now. (= It is possible that he is not reading a book now.)
Fourthly, they are more readily acceptable in the complement clauses of assertive predicates than in those of non-assertive predicates.