Modals to express degrees of certainty

This document discusses using modal verbs to express different degrees of certainty about statements in the present, past and future. It outlines modal verbs that can be used to speculate affirmatively or negatively about whether something is, was or may be true in the present (must be/can't be, may be/may not be), past (must have been/couldn't have been, may have been/may not have been) and future (should, may/may not).