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