Modals of Necessity & Obligation

Use modals of necessity and obligation to say that something is necessary, required, forbidden, or not necessary.

Have To

Use have to when something is necessary because of a rule, situation, job, or outside requirement.

Tense	Example
present	I have to wear glasses for reading.
present, third person	She has to work on Saturdays.
past	We had to wait for an hour.
future	I will have to call them tomorrow.

Use do, does, and did in questions and negatives.

Do you have to work today?

She does not have to wear a uniform.

Did you have to pay?

Must

Use must when the speaker feels something is necessary or gives a strong instruction.

I must remember to call Eva.

You must see this film. It is excellent.

Applications must be received by Friday.

For past obligation, use had to, not must.

Correct: I had to leave early yesterday.

Incorrect: I must leave early yesterday.

Mustn’t And Don’t Have To

These two forms have very different meanings.

Form	Meaning	Example
mustn’t	do not do it	You mustn’t tell anyone.
don’t have to	it is not necessary	You don’t have to come with me.

Needn’t

Needn't also means that something is not necessary, but it is more common in British English and formal contexts.

You needn't bring food. We have plenty.

Common Mistakes

Avoid	Use
She have to leave.	She has to leave.
Do I must pay?	Do I have to pay?
I must worked late yesterday.	I had to work late yesterday.
You don’t have to smoke here.	You mustn’t smoke here.

Practice

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Maria ___ to start work at 8:30 every day.

Summary

Use have to for rules, facts, and outside requirements. Use must for strong personal necessity or formal instructions. Use had to for the past. Remember the important difference: mustn't means forbidden, but don't have to means not necessary.