Modal Verbs use
In this topic, we use must, have to, don’t have to, and mustn’t to talk about obligation, necessity, no necessity, and prohibition.
Modal Verbs Form
Must is a modal verb. After must and mustn’t, we use the base form of the verb (V1) without to. Have to is a different structure: it is also followed by a verb, but the form of have changes: have to, has to, had to.
must + V1
mustn’t + V1
have to / has to / had to + V1
don’t / doesn’t / didn’t have to + V1
Modal Verbs Rule
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Must is used for a strong necessity or obligation.
It often shows the speaker’s own decision, strong feeling,
or a very important rule.
I must talk to her today.
Passengers must wear seat belts.
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Have to is usually used when the obligation comes from outside:
from rules, schedules, work, school, a situation, or another person.
I have to get up at six for work.
She has to show her ID at the entrance.
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For past obligation, we usually use had to.
We normally do not use must to talk about obligation in the past.
✅ We had to leave early yesterday.❌ We must leave early yesterday.
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Mustn’t means prohibition: it is not allowed.
It is not just advice, but a rule, a prohibition, or a very strong instruction not to do something.
You mustn’t park here.
Students mustn’t use their phones during the test.
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Don’t have to means no necessity.
This is important: don’t have to does not mean prohibition.
You can do the action, but it is not necessary.
✅ You don’t have to come early. (it is optional)✅ You mustn’t come early. (it is not allowed)
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After must and mustn’t, we use only V1 without to.
After have to, we also use the main verb in its base form,
but the structure itself includes to.
❌ You must to go now.✅ You must go now.✅ We have to leave now.
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Must does not change with the subject: I must, she must, they must.
But have to changes: I have to, he has to, we had to.
✅ He must wait.✅ He has to wait.❌ He have to wait.
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In questions with must, the verb comes before the subject:
Must I ...?
But in everyday English, questions with have to are often more natural:
Do I have to ...?
Must I finish this now?
Do we have to book in advance?
Modal Verbs Negation
There are two different kinds of negative meaning in this topic. Mustn’t = it is not allowed, a prohibition. Don’t / doesn’t / didn’t have to = it was / is not necessary, so there is no obligation.
mustn’t + V1
don’t / doesn’t / didn’t have to + V1
Modal Verbs Questions
Questions with must are formed without do. Questions with have to are formed with do / does / did.
Must + subject + V1?
Do / Does / Did + subject + have to + V1?