The meaning of must is be commanded or requested to. Must is a modal verb. To be forced to, as by convention or the requirements of honesty: To be compelled to in order to fulfill some need or achieve an aim: If used with a negative infinitive it indicates obligatory prohibition. When youre taking kayaking lessons, a lifejacket is a must. Used as an auxiliary to express obligation or compulsion: It is followed by the base form of a verb. We usually use must to talk about obligations which come from the speaker and we generally use have (got) to when we refer to obligations that come from outside the speaker. You must pay your dues. In this sense, must does not form a negative. You use must to indicate that you think it is very important or necessary for something to happen. How to use must in a sentence. You use must not or mustnt to … A must is something thats so important it cant be missed or ignored. Must is a modal verb that expresses necessity, obligation, or strong recommendation. We must hurry if were to arrive on time. I must say, that is a …