... lot of exams coming up soon, and he is also working on a term paper. He must have been studying at the library. Unit 5: Possibility and Probability Perfect Modals I. Possibility: May, Might & ... In negative sentences both must not and don’t have to (also don’t need to and need not) are used. Must not is used when the speaker expects or orders ... or could notImpossibility (negative only)Ex: That couldn’t be true!Note: We often use phrases like I think, I should think, I suppose, I guess, I reckon, etc…before may, might and could to...