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Must – modal auxiliary

Must is a modal auxiliary verb that we use primarily to express obligation, deduction and strong recommendations. Let’s take a closer look at this modal auxiliary verb.

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!

To deduct or express what we believe to be certain

  • 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.

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.
cursos de inglés

The difference between must and 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.

Lesson #11: Must


  • Dad, can I go1 to Jake’s house tonight?
  • Have you finished all your homework?2 Because you must3 finish it by Friday4 otherwise you won’t5 pass you exams this semester.
  • Well, I’ve done most of it.6
  • As I said before, you must finish all your homework before going out tonight.7
  • Do I really have to?8
  • Look, you’ve still got some time.9 Try to do as much as you can in the next couple hours, then I’ll10 let you go out with your friend, Jake.
  • Ok, thanks dad.
  • No worries, son.


  1. Dad, can I go to Jake’s house tonight? The modal auxiliary verb can is used for interpersonal usage.
  2. Have you finished all your homework? The present perfect is used here to talk about an action that happened in the past, and only recently. The answer is unknown, therefore, the present perfect is used instead of the past simple. We’ll go into the present perfect further into the course.
  3. Because you must finish it by Friday. The modal auxiliary verb must is used to express obligation.
  4. Friday. Days in English are always capitalised, that is, we always use a capital letter with the days of the week. I.e., Monday, Tuesday, Wednesday, Thursday etc.
  5. Otherwise you won’t pass your exams this semester. Will in the negative is will not, or the contracted form won’t. The negative of shall is shan’t. Will and won’t are much more common.
  6. Well, I’ve done most of it. The present perfect (have/has + past participle) is used here, I’ve done. The contraction is ‘ve. The following examples are contractions of have:  I’ve, you’ve, he’s, she’s, it’s, we’ve, you’ve, they’ve.
  7. You must finish all your homework. Must is used to show obligation.
  8. Do I really have to? The main auxiliary verb have is used with to to show obligation. This form is basically the same as must.
  9. You’ve still got some time. The main auxiliary have is used to show possession, but it’s common in English to use the form have + got. I.e., I’ve got a car. Sonia has got a nice smartphone. We’ve got two kids.
  10. I’ll let you go out with your friend, Jake. I’ll is the contracted form of I will. Other contractions of will are: I’ll, you’ll, he’ll, she’ll, it’ll, we’ll, you’ll (plural), they’ll.

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