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Friday, 25 October 2013

Have to/ has to/ don't have to / doesn't have to / Must/ mustn't

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)
Look at these examples in the simple tense:
subjectauxiliary verbmain verb haveinfinitive (with to)
+Shehasto work.
-Ido nothaveto seethe doctor.
?Didyouhaveto 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.

We use 'don't have to' to talk about things we have a choice about, things we aren't obliged to do. 

Subjectdon't / doesn't have tobase form of verb
don't have towear a uniform.
get up early on Sundays.
study hard!
doesn't have to

Must (subjective obligation)

We often use must to say that something is essential or necessary, for example:
  • 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").
Look at these examples:
subjectauxiliary mustmain verb

Use of Must

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

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").
Look at these examples:
subjectauxiliary must + notmain verb
Imustn'tforgetmy keys.
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:
  • 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)

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