Modals must/have to

Obligation and prohibition, and the gap between them.

S + must + V1 | S + have to/has to + V1

must and have to express obligation; must often signals a personal conviction or rule, have to an external requirement. The key trap: must not means prohibition (not allowed), while don't have to means no necessity (free not to do it) — the two are not interchangeable.

Examples

  • Drivers must carry a valid CDL.
  • You have to submit the POD within 24 hours.
  • You don't have to attend; it's optional.

  • You must not attend when it's optional.

    You don't have to attend when it's optional.

    must not means prohibition, not absence of necessity; "not required" is don't have to.

Common mistakes

  • Confusing must not with don't have to: You must not come today → You don't have to come today (when attendance is optional, not prohibited).
  • have to not agreeing with the subject in Present Simple: she have to sign → she has to sign.
Practice