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:

	subject	auxiliary verb	main verb have	infinitive (with to)
+	She		has	to work.
-	I	do not	have	to see	the doctor.
?	Did	you	have	to go	to 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.

Negative

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

Subject	don't / doesn't have to	base form of verb
I You We They	don't have to	wear a uniform. get up early on Sundays. study hard!
He She	doesn't have to

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").

Look at these examples:

subject	auxiliary must	main verb
I	must	go	home.
You	must	visit	us.
We	must	stop	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.

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:

subject	auxiliary must + not	main verb
I	mustn't	forget	my keys.
You	mustn't	disturb	him.
Students	must not	be	late.

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)

http://www.englishexercises.org/makeagame/viewgame.asp?id=5140 NEW !

http://www.eclecticenglish.com/grammar/HaveTo1A.html NEW !

http://www.better-english.com/grammar/mustnt.htm NEW !

http://www.englishclub.com/grammar/verbs-modals-have-to-must-not-quiz.htm

http://www.gigers.com/matthias/schule/rfe2-16_mustnt.html

http://www.better-english.com/grammar/mustnt.htm