Have to is NOT an auxiliary verb (it uses the verb have as a main verb). We include have to here for convenience.

Must is a modal auxiliary verb.

In this lesson we look at have to, must and must not, followed by a quiz  to check your understanding.

have to for objective obligation

We often use have to to say that something is obligatory, for example:

  • Children¬† have to¬†go to school.
Note that we can use the have to expression in all tenses, for example: he has to, he had to, he has had to, he will have to

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 basic structure for have to is:

subject+auxiliary verb+have+to-infinitive

Look at these examples in the Present Simple tense:

 subjectauxiliary verbmain verb
have
to-infinitive 
+She hasto work. 
-Ido nothaveto seethe doctor.
?Doyouhaveto goto school?

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.

In each of the above cases, the obligation is not the subject's opinion or idea. The obligation comes from outside.

We can use have to in  all tenses, and also with modal auxiliaries. We conjugate it just like any other main verb. Here are some examples:

 subjectauxiliary verbmain verb
have
to-infinitive 
Past SimpleI hadto workyesterday.
Present SimpleI haveto worktoday.
Future SimpleIwillhaveto worktomorrow.
Present ContinuousSheishavingto wait. 
Present PerfectWehavehadto changethe time.
modal mayTheymayhaveto doit again.

must  for 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 basic structure for must is:

subject+auxiliary verb
must
+main verb
base

The main verb is always the same form: base

Look at these examples:

subjectauxiliary verb
must
main verb
base
 
Imustgohome.
Youmustvisitus.
Wemuststopnow.
Like all auxiliary verbs, must cannot be followed by to. So, we say:
  • I must¬†go¬†now.
    not  I must to go now.

Use of must

In general, must expresses personal obligation. Must expresses what the speaker  thinks is necessary. Must is subjective . Look at these examples:

  • 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.

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.

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  for 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 basic structure for must not is:

subject+must not+main verb

The main verb is the base verb.

We often contract must not to mustn't.

Look at these examples:

subjectauxiliary
must + not
main verb 
Imustn'tforgetmy keys.
Youmustn'tdisturbhim.
Studentsmust notbelate.

NB: like all auxiliary verbs, must CANNOT be followed by to . So, we say:

  • You¬†mustn't ¬†arrive late.
    not  You mustn't to arrive late.

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.

sources : Original Link