Must and have to both express necessity or obligation, so learners often confuse them. The key difference is the source of the obligation: must usually reflects the speaker’s mind, while have to usually reflects an outside rule, situation, or requirement. In many contexts, both can be correct, but the emphasis changes.

Must is a modal verb used with a base verb and no do support. It is common for personal necessity, strong insistence, and formal obligations, especially in legal, official, or written language. In questions, Must I do this? sounds formal, and in negative form, mustn’t means prohibition rather than lack of obligation.

Idea	Example
Speaker necessity	I must leave now.
Formal obligation	You must wear a seat belt.
Prohibition	You must not enter.

Have to is a lexical verb pattern with to plus an infinitive, so it uses auxiliaries and do support in questions and negatives. It usually means external necessity, practical obligation, or something required by a rule, schedule, or situation. In informal American speech, have to is often more common than must, and have got to is a colloquial alternative.

Idea	Example
External necessity	I have to work early tomorrow.
Rule based obligation	Students have to show ID.
Colloquial form	I have got to go now.

Must can also express a very strong recommendation from the speaker, while have to usually stays closer to practical necessity. This difference is often about tone: must sounds more forceful, and have to sounds more neutral or situational. The boundary is flexible, so either form may appear when the obligation is factual rather than personal.

Idea	Example
Strong recommendation	You must try this soup.
Practical necessity	You have to finish the form first.
Either form	We must leave at noon.

Negation creates one of the biggest learner traps. Mustn’t means prohibition, so it says that an action is not allowed. Don’t have to and doesn’t have to mean there is no obligation, so the action is optional.

Idea	Example
Mustn’t	You mustn’t park here.
No obligation	You don’t have to pay today.
Optional action	She doesn’t have to attend.

When the obligation comes from the speaker, choose must. When the obligation comes from a rule, fact, or outside situation, choose have to. This simple source test solves many sentences, even when both forms are possible.

Idea	Example
Internal source	I must call her.
External source	I have to call her at work.
Source test	If the rule is outside you, use have to.

Must in questions, as in Must I wait, sounds formal and careful. Do I have to wait is the more common everyday question because have to behaves like a normal verb phrase. For register, must is frequent in formal and legal writing, while have to is common in speech and daily conversation, especially in American English. For a broader view of verb behavior, compare Auxiliary Verbs and Modal Verbs.

Region	Word or Phrase	Regional Definition	Example
Formal	Must	It is common in official, legal, and careful writing.	You must sign here.
Everyday	Have to	It is common in ordinary conversation and practical speech.	I have to go soon.
American	Have to	It is often preferred in informal American English.	We have to leave now.
Colloquial	Have got to	It is a casual alternative with the same basic meaning.	I have got to run.

Must and have to both mark necessity, but they point to different sources of obligation. Must highlights the speaker’s view, while have to highlights outside pressure or practical requirement. The safest choice is to ask where the obligation comes from, then match the verb to that source.