have to, must

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.

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:

Look at these examples in the Present Simple tense:

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:

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:

The main verb is always the same form: base

Look at these examples:

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.

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:

The main verb is the base verb.

Look at these examples:

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.