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I have heard people say that "have to" is a modal verb. Other people have told me it is not. Why exactly is "have to" a modal verb? Or why exactly is it not?

I have also heard that it is a periphrastic modal verb. Is a periphrastic modal verb a modal verb?

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Have to is not a modal verb, it is not even an auxiliary verb. In the have to structure have is a main verb:

Examples:

Declarative sentence:

I [do] have to go

Questions:

Do I have to go?

Negatives:

I don’t have to go.

Here, as we can see, the have to structure is used with the auxiliary do. This is not something specific to a modal. A modal would never occur next to an auxiliary or to another modal.

He can have to help find solutions.

Here, we see the occurrence of the have to structure next to the modal can.

Although we can name this structure Periphrastic modal if we wish, I would say that actually it is not a modal verb.

  • I think your answer needs a bit more unpacking for learners to be able to understand exactly what you're showing them (i.e. how do those examples show that have to isn't a modal verb?) – Araucaria Mar 28 '17 at 14:38
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There is a fixed list of modal verbs and semimodals.

Modal verbs: can, could, may, might, shall, should, will, would, and must.

Semimodals: ought + to-infinitive, have + to-infinitive, be able + to-infinitive, dare and need, had better (that is mostly classified as a compound verb), used + to-infinitive or used to + do-insertion.

While "have (got) to" isn't a modal verb, "have" is an auxiliary verb along with "be" and "do".

"Have to" or "have got to" (British English) also carries a meaning of something obligatory in the same way modal verbs do:

  • I must go to school.
  • I should go to school.
  • I have to go to school.

The way grammar says it:

"Have" is an auxiliary verb. Modal verbs are followed by infinitives without "to". In "have to", "have" is the main verb that follows a to-infinitive.

  • Have (got) to can either follow an auxiliary verb or not.

In the Simple Past and Simple Present it doesn't follow an auxiliary verb.

  • I have to read.
  • She had to play.

In the Future Simple it is preceded with an auxiliary verb "will".

  • I will have to go.

In the negative sentences it always follows an auxiliary verb.

  • I didn't have to go there. (do)
  • I won't have to be there. (will)
  • I don't have to sign this. (do)

In the interrogative sentences an auxiliary verb comes first and is followed by a subject, which is followed by "have to".

  • Did you have to go to school yesterday? (do)
  • Do you have to read this? (do)
  • Will you have to be there tomorrow? (will)

In fact we conjugate "have to" in the same way we conjugate any main verb.


Note: "have to" and "must" are both used to express a strong obligation. The difference is that with "must" this usually means that some personal circumstance makes the obligation necessary (and the speaker almost certainly agrees with the obligation.) and with "have to" this usually means that some external circumstance makes the obligation necessary.


Extra note: Remember that in the negative sentences "do not have to" suggests that someone is not required to do something while "must not" suggests that you are prohibited from doing something.

  1. It is a false assumption to say that "have to" is a modal verb of obligation.
  2. It is a false assumption to say that "have to" is a "have to + infinitive". In fact it is an auxiliary verb "have" plus a to-infinitive.

However, it is accepted by some dictionaries and sources to define "have to" as a modal verb. (Oxford Learners Dictionary, Perfect English Grammar, Ginger, LinguaPress, ThoughtCo, Woodward English)

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In a broader sense, verbs are generally of two types - Auxiliary verbs and Lexical verbs.

Auxiliary Verbs are further classified into two - Modal auxiliary verbs and Non-Modal Auxiliary Verbs.

So to know whether have to is a modal verb or not, we have to first know whether it's a lexical verb or an auxiliary verb.

LEXICAL VERB vs AUXILIARY VERBS

A good device to distinguish an auxiliary verb from other lexical verbs is NICE. It stands for (N)egation, (I)nversion, (C)ode and (E)mphasis.

  • NEGATION - (Primary Verb negation)

A lexical verb takes do support for negation, generally, but an auxiliary verb doesn't.

I don't want it any more. [want - A lexical verb, needs do-support]

He is not a thief. [is - is an auxiliary verb, and so don't need any do-support]

But have to behaves little differently in case of negation. For negation, it can take do-support, or it can go without it. Both constructions are fine.

He doesn't have to be that cautious.

We have not to repeat it again.

You know, you have to not be somebody who takes all of their anger.

  • INVERSION -

For inversion also, a lexical verb takes do-support, but an auxiliary verb doesn't.

Do you want another drink? [The lexical verb - want - takes do-support for inversion]

Is he a film star? [The auxiliary verb - is - doesn't take do-support for inversion]

Let's check how have to behaves during INVERSION -

Do I have to study hard?

I have to study hard -> * Have to I study hard? [INCORRECT]

I have to study hard -> * Have I to study hard? [INCORRECT]

So in this construction, have to behaves like a lexical verb.

  • CODE -

It's like ellipsis. The Verb Phrase (VP) of a clause is reduced. And the reduced portion is recovered from the context. In the following sentences the reduced or elliptical position is denoted by a "___". An auxiliary verb can take part in such way, but a lexical verb can't.

I can drive this car, and Bill can ____ too. [The elliptical portion - drive this car - is denoted by a "___"]

This sentence is correct because the auxiliary verb - can - take part in such constructions.

I want to drive that car, and my friend wants ___ too. [INCORRECT - because lexical verb - want - can't take part in such constructions]

Now let's examine have to with such constructions -

Maybe I'll go over there if I really have to ___. [In the gap we can write go over there]

Like an auxiliary verb, have to can take part in such constructions.

  • EMPHASIS -

The emphasis is on the auxiliary verb, when there is no auxiliary verb we need a dummy do for emphasis or stress.

I never imagined that he could pass the test, but he COULD pass the test. [STRESSED]

I never imagined that he passed the test, but he PASSED the test. [NO STRESS]

I never imagined that he passed the test, but he DID pass the test. [STRESSED]

Now let's examine how have to behave in such cases.

I have to win the race, and I DO have to win the race. [STRESSED]

The verb have to needs do support for stress. It's a property of a lexical verb.

Apart from NICE, there are other ways to determine whether a verb is an auxiliary verb or a lexical verb.

  1. Placement of adverbs
  2. Negative inflection and reduced form

      1. Placement of adverbs -

Frequency adverbs (like always, often etc) and modal adverbs (like possibly, certainly etc) generally precedes lexical verbs, and follows auxiliary verbs.

He is always after money. [always follows an auxiliary verb - is]

He always wants money. [always precedes a lexical verb - want]

Now check how have to takes such adverbs -

He always have to ask Charlie.

You have to always evolve.

You have always to talk.

    1. Negative inflection and reduced form -

Well, like auxiliary verb, have to has a reduced form - haven't to.

Some wives earn a lot of money and so their husbands haven't to work.

She said I can't tell you, I haven't to tell you!

So judging from the above, it's clear that have to has both the property of a lexical verb and the property of an auxiliary verb. So some grammarians (for example Quirk et el. in A Comprehensive Grammar of English Language) call have to a Semi-Auxiliary Verb. Some grammarians don't call it that. For them, have is the verb, and it have dual property. It can either be a lexical verb or an auxiliary verb.

So even if it's an auxiliary verb, it's not a modal verb.

[To be continued - Discussion regarding why it's not a modal verb]

  • You might want to delete your post while you're editing it! A few points. (A) For some speakers have in the have to construction, does behave like an auxiliary verb. So these speakers say thing like Have we to be there tomorrow? and We've to be there by 5pm, haven't we. This is relatively rare, though. (B) We can have subclausal negation where the sentence has positive polarity but an embedded subordinate clause is negated. So the sentence You have to not respond to his provocations is grammatical ( - as is You have to not go there, although it is clunky). ... – Araucaria Apr 2 '17 at 13:44
  • ... Notice that this sentence has a negative polarity tag showing that the sentence is positive: "You have to not respond, don't you". So the not here belongs with respond not with have. (C) The word to takes part in code constructions - one of the reasons that Pullum and many others believe it's an auxiliary verb: "I have to go there and my friends have to ___ too" (this sentence is grammatical - your example isn't but that's because of either, which is a negative polarity item. – Araucaria Apr 2 '17 at 13:46
  • I would think about showing (i) that it doesn't behave like an auxiliary for most speakers (ii) that it isn't a modal auxiliary for any speakers (iii) that the to does not belong with have - it belongs with the following verb (i.e. the following VP). But I like the general angle your taking ... :-) [You might want to add that modal verb is generally used as a syntactic label, not a semantic one ...] – Araucaria Apr 2 '17 at 13:49

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