Modal verbs fine-tune certainty. For a weak, unsure guess — 'It ___ rain later, but I'm not sure' — which one fits?

"Might" expresses a weak, tentative possibility, making it the natural choice when someone is genuinely unsure whether something will happen, exactly matching the uncertainty signaled by "but I'm not sure" later in the sentence.

"Must" conveys strong logical certainty or obligation, far too confident a claim for someone hedging their prediction, and "will" states a firm, direct prediction about the future — both clash with the admitted uncertainty in the sentence.

English modal verbs form a whole scale of certainty from weakest to strongest — might, may, could, should, must — and choosing the right one lets speakers communicate precise shades of confidence without adding extra explanatory words.