Have to, Must

Must is a modal auxiliary verb.

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

In this lesson we look at these two verbs, followed by a quiz to check your understanding:

Have to (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:
I have to, I had to, I have had to, I 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 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.

In each of the above cases, the obligation is not the subject's opinion or idea. The obligation is imposed 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:

	subject	auxiliary verb	main verb have	infinitive
past simple	I		had	to work	yesterday.
present simple	I		have	to work	today.
future simple	I	will	have	to work	tomorrow.
present continuous	She	is	having	to wait.
present perfect	We	have	had	to change	the time.
modal (may)	They	may	have	to do	it again.

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.

 

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.