Must And Have To - The Modal Verbs

Must and have to have similar meanings in English. Both are used to express necessity. However, there are a few important differences between them.

Must is a modal verb.
  • Modal verbs do not take any endings like -s, -ed or -ing (never ‘musts’ or ‘musted’)
  • Modal verbs are followed by the base form of another verb (must do, must be)
  • Modal verbs make questions by inverting the subject and the modal verb (must you)
Examples with must:
  • Passengers must present their tickets to the ticket officer.
  • You must remember to submit the documents by the deadline.
Have to is not exactly a modal verb.
  • Have takes endings like -ing (having) although the present and past are irregular (have/has, had)
  • Have to is formed by using have + infinitive (have to do, have to be)
  • Questions with have to require do as an auxiliary verb (Do you have to…?)
Examples with have to:
  • I have to go to the grocery store on my way home from school.
  • They had to ask their classmates for help with the assignment.
  • Do you have to work late today?

The examples above show the syntactic differences between must and have to (syntax = rules for word order). Now let’s look at the differences in the meaning of these verbs.

Must is more commonly used to express obligation in formal writing, such as in printed rules and instructions. It is not common for people to speak to each other about necessity by using the word must (**see below). Notice how these examples resemble written rules rather than conversation:
  • All employees must wear an identification badge for security purposes.
  • Passengers must remain seated at all times.
  • Visitors must obtain a parking permit at the entrance.
  • Students must not eat or drink in the laboratory.
Have to is more commonly used express obligation or necessity in conversation. Have to can also be used in informal writing. Notice how these examples resemble informal conversation:
  • I have to leave work early to go to a doctor’s appointment.
  • Julie had to pay a fine for returning the books late.
  • Do we have to bring our books to class every day?
  • You don’t have to provide your name on the survey.
The negative form of must and have to
The negative form of must is must not. Must indicates obligation, and must not indicates prohibition.
  • Visitors must obtain a parking permit at the entrance.
    • This is a rule. Visitors have no choice.
  • Visitors must not feed the animals.
    • This is also a rule. Visitors have no choice.
The negative form of have to is don’t/doesn’t have to. Didn’t have to is used for the past and will not have to for the future. This is where things can get a little tricky! Have to indicates a necessity or requirement, but don’t have to indicates a choice. In other words, it’s not necessary. You can decide if you want to do it or not.
  • I have to go to the grocery store on the way home from school.
    • I need to do this.
  • I don’t have to go to the store today.
    • I can go if I want to, but I don’t need to. I don’t really need anything.
  • You don’t have to provide your name on the survey.
    • You can provide your name if you want to, but it’s not required. It’s your choice.
The negative form of have to is a common source of mistakes among students. Remember that have to and must have similar meanings. But the negative forms of these verbs don’t have the same meaning. Here are some examples of common mistakes.
  • Incorrect: You don’t have to smoke in the restaurant.
    • This is a very unlikely sentence. Remember that have to indicates a requirement. This sentence says that I am not required to smoke, but what restaurant would require smoking?
    • Correct: You must not smoke in the restaurant. OR You can’t smoke in the restaurant. OR Smoking is not permitted in the restaurant.
  • Incorrect: You don’t have to throw trash on the ground.
    • This sentence says that I am not required to throw trash on the ground (but I can if I want to.) This is also a very unlikely sentence.
    • Correct: You must not throw trash on the ground. OR Do not throw trash on the ground. OR Littering is prohibited.
**It isn’t common for people to speak to each other about necessity by using the word must. However, must is used in conversation in other contexts.
  • Deductions – Making deduction means using the knowledge you have to come to a conclusion. In the context, must is similar to ‘seems like’ and does not indicate an obligation or necessity.
    • Imagine you go to a restaurant with your friend, and she orders a whole pizza for herself. You might say:
      • You must be hungry!
    • Let’s say you planned to meet your friend at the library at 5:00, but she’s not there. You might think:
      • She must be late. OR
      • She must have forgotten.
  • Strong recommendations – In this context, must is used to add emphasis to a recommendation and does not indicate an obligation.
    • You must try this pecan pie!
  • Questions with irritation – In this context, the speaker is expressing frustration with someone’s behavior. This is not a question about necessity.
    • Must you turn the volume up so high?
    • To ask a question about necessity, it is more natural to use have to instead of must.
      • Do you have to go to class today?
        • NOT: Must you go to class today?