When do you use 'must' in English

Must is a modal verb that is followed with another verb without ‘to’ to express necessity, personal obligation, prohibition, certainty. Necessity or personal obligation We often use ‘must’ to say that something is necessary or very important to do. We don’t have much time. We must hurry. Certainty We often use 'must' when we are certain something is true. You must be tired after a long day at the office.