English Grammar Basic Grammar and syntax

Must vs have to/ are they really interchangeable?

Must vs Have to/ Are they really interchangeable?

Must is a modal auxiliary verb, and have to is a simple auxiliary that is used as a modal verb, but is not a modal verb. The reason we are learning it together is that both verbs have similar functions. In this article, I will help you master all the differences between must and have to, and how to use these verbs.

1. To talk about obligations/necessities

The most common uses of must and have to are to talk about obligations or needs (situations/things that are essential).


  • Use MUST to talk about an obligation that is internal.
    (When an obligation is internal (you understand the importance of something and know it is not the right thing to avoid it), use MUST to talk about it.
  • Use must to expresses what the speaker thinks is right or necessary to do.

Let’s take some examples of must!


  • I must attend this class. It will help me to pass the exam.
  • We must change our game plan. This does not seem to be working.
  • Students must have an ID card to sit in the exams.
  • You must wear a seat belt while driving.


  • It expresses an obligation that is external.
  • Expresses that speaker or the subject is forced or obliged to do something.
  • Expresses that event can’t be avoided. The speaker or the subject does not have any other option.

Examples of have to:

  • You have to work in the finance department.
  • I have to vacant the flat in the next 10 days. The court has ordered me to do so.
  • We have to return the books to the librarian.
  • Everyone has to wear casual clothes on weekdays. It’s been decided by the management.
  • She had to marry Jon because of her family pressure.

2. Must not and don’t have to

Las formas negativas de must y have to tienen un significado extremadamente diferente.


  • The negative form of must (must not or mustn’t) meaning“not allowed or prohibited.“
  • Mostly, the prohibition is external (comes from outside), but the speaker could prohibit themselves from doing something too.

Examples of must not:

  • You must not touch my phone.
  • You must not bring a phone to the examination.
  • He must not see that girl anymore.
  • I must not play games anymore. (personal decision)
  • You must not open the box until I tell you to do so.

NOTA: Mustn’t es una contracción de must not, pero no es muy común su uso tanto en inglés hablado como escrito.

Have to
Don’t have to = not necessary or required, you are not required or forced to do something.

The negative forms of have to are followings:

  • Don’t have to – Plural subject (I, you, we, they)
  • Doesn’t have to – Singular subject (He, she, it)
  • Didn’t have to – Both singular and plural

Nota: Don’t, does not y didn’t son contracciones de do not, does not y did not.

  • You don’t have to wait for me. (= You can wait for me, but it’s really not required/necessary.)
  • She doesn’t have to marry me. (= If she wants, she can…she is not obliged to do so.)
  • He did not have to do that job. He just did it for fun.
  • I don’t have to live here anymore, but I love this place.
  • Max does not have to apologize to her.

3. To give opinions/suggestions/recommendations

Tanto must como have to se utilizan para dar fuertes sugerencias, opiniones, recomendaciones o consejos. Casi no hay diferencia entre must y have to cuando los usamos para dar nuestra sugerencia, opinión o consejo.


  • You must talk to your family about the case.
  • Students must follow the people who have done something significant in society.
  • We must stop beating our kids.
  • He must stop hanging out with those boys. They are into terrible things.
  • You must listen to Gary Vee for once. Your life will not be the same after that.
  • You must try the South Indian food here. It is mindblowing.
  • We must stand against the bullies.

Have to

  • You have to talk to your family about the case.
    (It is a piece of very strong advice, probably stronger than MUST. Have to, here, gives the sense of an obligation, but it isn’t an obligation though. )
  • You have to change the way you think if you want to achieve something big.
  • She has to work on her communication skills.
  • Jim has to be nice to others. Everyone hates her.
  • You have to stop wasting time watching porn.
  • You have to listen to Gary Vee for once. Your life will not be the same after that.

4. To talk about possibility/certainty

Si está seguro de algo, use must o have to. Casi no hay diferencia entre must y have to cuando queremos hablar de la certeza o posibilidad de un evento del que tenemos mucha confianza.


  • You look ripped and bruised. You must be a fighter.
  • Make sure you take a blanket with you. It must be really cold there.
  • He generally doesn’t take a day off. He must be sick.
  • I think I offended Riya. She must be really mad at me.
  • Last night must have been amazing. You look elated.
  • There are beer bottles everywhere. They must have had a party last night.

Have to

  • It has to be the correct answer.
  • Somebody left a phone on my stairs. That has to be Monu.
  • This has to be the reason for her silence.
  • It has to be a murder. There are so many things leading us in that direction.

Nota: también podemos usar el verbo modal should para hablar de la posibilidad de un evento.

  • It should be the correct answer.
  • Somebody left a phone on my stairs. That should be Monu.

5. Must and have to in the past

  • MAY = used to refer to the present and the future time, can’t be used in the past to talk about an obligation
  • HAVE TO = along with the present and the future, it can also be used in the past


  • We must finish the work. (Present)
  • I must attend this class. (Present)
  • We must go there tomorrow. (Future)
  • She must not go on that trip next week. (Future)


  • We have to pay the rent now. (Present)
  • We will have to pay the rent next month. (Future)
  • We had to pay the extra money last month. (Past)
  • I had to quit my job because of the toxic environment. (Past)
  • Rony had to smoke that day. He’d lost a bet. (Past)

6. Modal verb

Must es un verbo auxiliar modal, y muchos estudiantes piensan que have to también es un verbo modal. Pero ese no es el caso.

TENER TO is a simple verb form made from the verb TO HAVE. It changes its shape based on number and time, which MUST does not.

  • I have to pay her back.
  • He has to pay her back. (change of the subject)
  • I had to pay her check. (change of the tense)

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