This document discusses different modal verbs used to express probability: - Will is used to express certainty that something will or will not happen in the future, such as "The sun will rise tomorrow." - Must is used to indicate something is happening or has happened in the present based on available evidence or reasons, like "The baby is crying. He must be hungry." - Should is used to suggest something is likely or probable but not certain, for example "They left an hour ago. They should be here by now." The document provides examples of will, must, and should being used to convey different levels of probability or certainty.