Must and have to both express obligation or necessity. In many situations, they are close in meaning, but speakers often feel a small difference. Must often sounds more personal or stronger, while have to often sounds like a rule, situation, or outside requirement.

Must is often used when the speaker gives the rule, feels a strong need, or wants to give firm advice. It can also show the speaker’s opinion about what is necessary. In everyday speech, some speakers use have to more often than must for simple obligation.

Rule
Use must when the obligation comes from the speaker or from the speaker’s judgment.
Use must for strong advice when the speaker wants to sound firm.
Use must for personal rules that the speaker sets for themself.

Have to is often used when the obligation comes from outside the speaker. It is common for laws, school or work duties, schedules, and practical situations. In everyday English, have to is very common for ordinary obligation.

Rule
Use have to when another person, a system, or a situation creates the obligation.
Use have to for duties, timetables, and formal requirements.
Use have to when the necessity is practical rather than personal.

Have to changes form like a normal verb phrase. It uses have or has in the present, had in the past, and will have to in the future. Questions and negatives use do, does, or did.

Subject	Form
I	have to
You	have to
He	has to
She	has to
It	has to
We	have to
They	have to

In the negative, have to becomes do not have to, does not have to, or did not have to. In questions, the form is do have to, does have to, or did have to. Must does not use do in standard forms.

Subject	Form
Present negative	do not have to or does not have to
Past negative	did not have to
Present question	do have to or does have to
Past question	did have to
Future	will have to

Must can also express more than obligation. It is used for strong advice, for personal decisions, and for certainty when the speaker feels sure something is true. This certainty meaning is different from have to.

Word or Phrase	Definition
obligation	It expresses a strong need or duty that the speaker feels clearly.
advice	It can give firm advice from the speaker.
certainty	It can show that the speaker feels sure something is true.

Must not and do not have to have very different meanings. Must not means something is prohibited or not allowed. Do not have to means something is not necessary, but it is possible if the person wants to do it.

Rule
Must not means the action is forbidden.
Do not have to means there is no obligation.
These forms are not opposites in the same way, so speakers must choose carefully.

Questions with have to are common when asking about duties, rules, or practical needs. Questions with must are less common for ordinary obligation and can sound formal or direct. In many everyday situations, speakers prefer have to in questions.

Rule
Use have to questions to ask whether an outside obligation exists.
Must questions are possible, but they often sound formal, strong, or less usual in daily speech.
In conversation, have to is often the more natural choice for asking about necessity.

You can now choose between must and have to to express obligation and necessity. You can use must for personal, strong, or speaker-based meaning, and have to for outside rules and practical requirements. You can also form have to correctly in statements, negatives, and questions, and you can distinguish must not from do not have to.