Using must and have to

“Vistors must check-in at the office.”

The modal verbs “must” and “have to” both express necessity or obligation, but there are subtle differences in their usage. Continue reading to learn more about their differences and when to use them.

Key differences

• Source of obligation
  • Must
    • Often indicates an internal obligation or a strong personal feeling of necessity. It can also be used for rules and regulations, especially in formal contexts.  
    • The obligation often comes from the speaker’s perspective.  
  • Have to
    • Typically indicates an external obligation, such as a rule, law, or someone else’s authority.  
    • The obligation often comes from an outside source.  
• Formality
  • Must
    • Generally considered more formal.
  • Have to
    • More common in everyday spoken English.
• Negative forms
  • Mustn’t
    • Indicates prohibition (something is not allowed).  
  • Don’t/Doesn’t have to
    • Indicates the absence of obligation (something is not necessary).  
• Tense
  • Must
    • Primarily used in the present tense. It cannot be used in the past or future to show obligation.
    • When using the past, consider using “had to” or “had got to”. Examples: I had to finish before noon. (US) / He had got to finish before work. (UK)
    • When using the future, consider using “will have to”. Example: We will have to find another venue for the wedding.
  • Have to
    • Can be used in all tenses (had to, will have to). This is a crucial difference.

How to use them

• Must
  • “I must finish this report today.” (Personal obligation)
  • “Passengers must fasten their seatbelts.” (Formal rule)
  • “You mustn’t smoke in here.” (Prohibition)
• Have to
  • “I have to go to work.” (External obligation)
  • “She has to study for her exams.” (External obligation)
  • “They had to cancel the meeting.” (Past obligation)
  • “You don’t have to come if you don’t want to.” (Lack of obligation)  
  • “We will have to buy groceries tomorrow.” (Future obligation)

Key points to remember

• Use “must” for internal obligations and formal rules.  
• Use “have to” for external obligations and for expressing obligation in different tenses.  
• Pay close attention to the negative forms, as they have very different meanings.

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