Must/have to (Modals)

In this lesson, you will learn how to talk about obligation — things that are necessary or required.
You’ll also learn an important difference: mustn’t (prohibition) is not the same as don’t have to (no necessity).

1. What obligation means

Obligation means something is necessary or required (you cannot choose not to do it).

2. Using must

We use must to express a strong obligation. It can be the speaker’s strong instruction/advice (“You must see a doctor”), or it can sound like an official rule (signs/notices: “Employees must wear ID badges”).

Examples:

• “I must finish this report today.”
• “You must wear your ID badge in the building.”

(On a sign) “Visitors must wear safety helmets.”

3. Using have to

We use have to when the obligation comes from rules, laws, or external situations.

Form (important):

• Affirmative: I/You/We/They have to… / He/She has to…
• Negative: I don’t have to… / He doesn’t have to…
• Question: Do you have to…? / Does he have to…?

Examples:

• “I have to wear a uniform at work.”
• “We have to show our passports at the airport.”

Note: The past form of “have to” is had to for obligations in the past.

• I had to work late yesterday.
• I didn’t have to work last weekend.
• Did you have to pay for the ticket?

We don’t usually use must for the past; we use had to: ‘I had to leave early yesterday.’

4. Must vs. have to: meaning difference

Rule of thumb: must often sounds stronger/more personal (or more formal in notices); have to is common for facts, rules, or situations. In many everyday sentences, both are possible with little difference.

Compare how the source of obligation changes the meaning.

Examples:

• “I must study tonight.” → Personal decision.
• “I have to study tonight.” → The situation requires it.

5. Negative forms (important difference)

The negative forms have very different meanings.

Examples:

• “You mustn’t park here.” → It is not allowed.
• “You don’t have to come tomorrow.” → It is not necessary.

To express prohibition, we normally use mustn’t, can’t, or (be) not allowed to: “You mustn’t park here.” / “You can’t park here.” / “You’re not allowed to park here.” Don’t have to never means ‘forbidden’—it only means ‘not necessary.’

6. Key takeaway

• Must shows strong, personal obligation.
• Have to shows external obligation.
• Mustn’t means prohibition.
• Don’t have to means no obligation.