must

The modern modal verb 'must' evolved from the Old English past tense form 'mōste' of the preterite-present verb 'mōtan', meaning 'to be allowed, to be able to'. This origin explains its lack of an -s in the third person singular (he must) and its use without 'to' before the main verb. The sense of obligation or necessity developed from the older meaning of 'had to' or 'was obliged to', which was inherent in its past tense origin. Over time, this past tense form supplanted the present tense forms, solidifying its role as a modal verb expressing compulsion or inevitability.

The word 'must' is a single, indivisible morpheme in modern English. It functions as a modal auxiliary verb.