Have to (objective obligation)
We often use have to to say that something is obligatory, for example:
Children have to go to school.
Structure of Have to
Have to is often grouped with modal auxiliary verbs for convenience, but in fact it is not a modal verb. It is not even an auxiliary verb. In the have to structure, "have" is a main verb. The structure is: subject + auxiliary verb + have + infinitive (with to)
Use of Have to
In general, have to expresses impersonal obligation. The subject of have to is obliged or forced to act by a separate, external power (for example, the Law or school rules). Have to is objective. Look at these examples:
In France, you have to drive on the right.
In England, most schoolchildren have to wear a uniform.
John has to wear a tie at work.

Must (subjective obligation)
We often use must to say that something is essential or necessary, for example:
I must go.
Structure of Must
Must is a modal auxiliary verb. It is followed by a main verb. The structure is:
subject + must + main verb
The main verb is the base verb (infinitive without "to").
Use of Must
In general, must expresses personal obligation. Must expresses what the speaker thinks is necessary. Must is subjective. Look at these examples:
It is sometimes possible to use must for real obligation, for example a rule or a law. But generally we use have to for this.
I must stop smoking.
You must visit us soon.
He must work harder.
In each of the above cases, the "obligation" is the opinion or idea of the person speaking. In fact, it is not a real obligation. It is not imposed from outside.
We can use must to talk about the present or the future. Look at these examples:
I must go now. (present)
I must call my mother tomorrow. (future)
We cannot use must to talk about the past. We use have to to talk about the past

Must not, Mustn't (prohibition)
We use must not to say that something is not permitted or allowed, for example:
Passengers must not talk to the driver.
Structure of Must not
Must is an auxiliary verb. It is followed by a main verb. The structure for must not is:
subject + must not + main verb
The main verb is the base verb (infinitive without "to").
Use of Must not
Must not expresses prohibition - something that is not permitted, not allowed. The prohibition can be subjective (the speaker's opinion) or objective (a real law or rule). Look at these examples:
I mustn't eat so much sugar. (subjective)
You mustn't watch so much television. (subjective)
Students must not leave bicycles here. (objective)
Policemen must not drink on duty. (objective)
We can use must not to talk about the present or the future:
Visitors must not smoke. (present)
I mustn't forget Tara's birthday. (future)
We cannot use must not to talk about the past. We use other structures to talk about the past, for example:
We were not allowed to enter.
I couldn't park outside the shop.

Have to, Must, Must not Quiz
1 Yesterday I must/mustn't/had to finish my Geography project.
Click for answer had to
2 She will must/have to/has to wait in line like everyone else.
Click for answer have to
3 All employees must be/mustn't/have to on time for work.
Click for answer must be
4 We have to not/must/mustn't forget to take the chicken out of the freezer.
Click for answer mustn't
5 If you are under 13 you have/must/musn't to get your parents' permission.
Click for answer have
6 Your daughter may have to/had to/must try on a few different sizes.
Click for answer have to
7 The doctor must/mustn't/have to get here as soon as he can.
Click for answer must
8 Do you have to/must/mustn't work next weekend?
Click for answer have to
9 Bicyclists mustn't/must/has to remember to signal when they turn.
Click for answer must
10 Angela, you mustn't/must/have to leave your clothes all over the floor like this.
Click for answer mustn't