This document discusses modal verbs used to show possibility in English, including may, might, could, and must. May and might show present or future possibility and are less definite than could. Could shows something is possible in the present or future. All three can be used in the past with "have" to show past possibility. Might not and could not express that something probably is not or will not happen. Examples are provided to illustrate the usage of each modal verb of possibility.