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Modal auxiliaries are helping verbs that connect with normal/ordinary verbs to express a meaning, ask a question or negate. Modal auxiliaries are never used with the main auxiliaries; ‘be‘, ‘have‘ and ‘do‘, and do not make sense on their own, therefore, they must be connected with a normal verb in order to make sense. Modal verbs also never change form (they cannot be conjugated). Dictionary definition according to Merriam Webster: ”an auxiliary verb (such as can, would, might, may) that is characteristically used with a verb of predication and expresses a modal modification and that in English differs formally from other verbs in lacking -s and -ing forms”.  Here, we are going to elaborate on modal auxiliaries, must. (Note that modal auxiliaries are never followed by ‘to’ before the infinitive, except with, ‘need to‘, ‘ought to‘ and ‘dare to‘).

(1) The main use of must is to express obligation. 
  • They must do what is intended for the project.
  • Amy must not cross at a red light.
  • She mustn’t tell anyone about our secret!
(2) We use must to deduct or express in what we believe to be a certainty.
  • He arrived late to work. The boss must have been annoyed.
  • Spain gets really hot so it must be hard during the summer months.
  • Our company has increased its income by 20%, therefore, it must be going really strong.
(3) We use must to express strong advice, suggestions or recommendations.
  • Listen, you must listen to my friend because he knows what he’s talking about.
  • You must see the new bar that just opened, it’s amazing!
  • The food there is so delicious, you must go one day.

Difference between: must vs. have to

The modal auxiliary verb, must can be substituted for, have to with very little difference in meaning. Have to is only a little more formal than must.

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