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A friend of mine is telling me that " have to " is not a modal verb, even thought I have learned that it is.

So is the " have to" a modal verb or not? If not which are the reasons?

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  • As it says here: Modals are different from normal verbs: 1: They don't use an 's' for the third person singular. 2: They make questions by inversion ('she can go' becomes 'can she go?'). 3: They are followed directly by the infinitive of another verb (without 'to'). Noting particularly that final point, I think we have to accept that have to doesn't make the cut as a modal verb. Jan 15 '16 at 15:04
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The term modal verb is problematic in English. The central modal verbs in English are:

  • can, could, may , might, shall, should, will, would, must

Syntax

These are a group of verbs which are distinguished by their syntax, more than by their meaning:

They are always followed by a verb in the plain form.
We never find two of them together.
They are always the first verb in the verb phrase.
They don't occur with the dummy auxiliary DO.
They don't inflect for person.
They have no non-finite forms (no forms like * canning or * to can exist).
They are always auxiliary verbs, never lexical verbs.

This last sentence means that, like the auxiliary verbs DO, BE AND HAVE, they exhibit NICE properties. NICE is an acronym for:

  • Negation - they contract with not to form negatives:

I can't come

  • Inversion - they often invert with Subjects:

Can I come?

  • Code - They can occur stranded at the end of a sentence when a main verb is missing:

I don't know if I can. [come]

  • Emphasis: They may be stressed to give positive emphasis to an utterance

I can dance!

Syntax versus meaning

Modal verbs have meanings related to modality. They tells us about things like potentiality and necessity in relation to ideas like obligation (deontic modality) or what we can deduce (epistemic modality). Now, the modal verbs in English are a syntactic class of word. Unfortunately they have a title which is all about semantics. This can cause confusion. The English modal auxiliary verbs are interesting and important because of their syntax, but they're also interesting and important because of their semantics too. When we think purely about the syntax then have to isn't very much like a modal verb at all. For example:

  1. Do I have to give him his bike back?
  2. I don't like having to bow and scrape
  3. I don't want to have to punch you.
  4. She must have to be there very early

Here we see have to occurring with do; a non-finite form, having to; a non-finite plain form of the verb in the string to have to; have to co-occurring with a modal verb. We can also show that, in fact, have to isn't even a verb. It's two completely separate words. The to in have to belongs with the following verb, not with the verb have. Consider:

  • You have only to ask.
  • You have, in my opinion, only to ask.

Here we see the phrase to ask being modified by the adverb only. In the second example we see a parenthetical phrase separating these two words.

However, from a meaning point of view, the string have to seems to be used to express many of the same types of modality as we see with modal verbs.

  • You have to be here by 9 am (deontic modality)
  • It has to be in one of these three boxes (epistemic modality)

People who describe have to as a semi-modal or periphrastic modal, use this term because have to is an important term for expressing ideas about epistemic and deontic modality in English. Now, syntacticians like myself may get shirty about this - have to is nothing like the real 'modal verbs' - but that's our own fault for choosing a dumb semantic title for a syntactic class of word. Other languages have verbs for expressing modality which behave exactly like other normal verbs. They are occasionally referred to as modal verbs, because they are used to express ideas about modality. It's perfectly reasonable therefore to continue to use a similar terms for normal English verbs that express ideas about modality too. The only thing is that it is important to remember that the English modals - as apart from the semi-modals - are a syntactic class of word. The semi-modals are a group of words which are used to express semantic modality, but don't have the same syntax as the modal auxiliaries.

Conclusion

Have to is not a modal verb in the sense that is not in the same syntactic family of words such as can, must or will. However, have to definitely is used to express ideas about modality. Some people refer to English verbs that express ideas about modality but don't share the same syntactic properties as can or must as "semi-modals" or "periphrastic modals". So we can consider have to a semi-modal, if we wish. This is fine as a semantic term for these words. Some syntacticians don't like this. Tough. If there's any blame here for the confusion, it must be laid at the feet of us syntacticians who chose a semantic name for a syntactic class of word!

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  • "If there's any blame here for the confusion, it must be laid at the feet of us syntacticians who chose a semantic name for a syntactic class of word!" this summarises the root of the problem very well
    – Some_Guy
    Dec 10 '19 at 12:27
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It's not an Official Modal Auxiliary Verb with Operator Superpowers, like you learned in school:

Instead, it's what's called a Periphrastic modal. They're like sidekicks to the Official Modals.
See, Modals have limitations (like Kryptonite for Superman):

  1. A Modal may not be inflected. (no -s on 3SgPres forms, for instance)

    • *Today he's working; yesterday he musted work, but didn't.
  2. A Modal must be the first auxiliary verb in a verb phrase it appears in.

    • *I used to can do that, but I can't any more.

Together, these mean that Modals lack the flexibility to appear in many English constructions.

So, English has cleverly innovated a number of idiomatic constructions that mean the same thing as modals, but are not subject to their restrictions. Most modals have such paraphrases:

  • must = have to
  • will = be going to, be to
  • should = be likely, be probable
  • may, might, can, could = be possible, be able

Since these are all idioms, their grammar varies.

Periphrastic modals have one other interesting use (nothing in language has only one function).
Modals and Negatives are both Operators and interact in idiomatic ways. So, a sentence like

  • You must not attend the ball.

can't mean that the addressee can decide to attend or not to attend, as they wish.
In order to express that meaning, a periphrastic modal comes in handy with the negative:

  • You don't have to attend the ball.
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Have to is a Modal Verb only when it is used in the sense of must. Whereas, must has no preterite form, had to functions to indicate the past action.

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No, it is not a modal verb, here's a full list of modal verbs:

can could may might will would must shall should ought to

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    How about "need" and "dare"? Why is ought to a member of that class when it is followed by an infinitive with to? Jan 15 '16 at 14:46
  • @Araucaria: Ought we consider this a valid question? Or need we to insist that the infinitive marker always be present? Jan 15 '16 at 14:59
  • @Araucaria Oh yes, sorry if forget to include 'need' and 'dare'.
    – shotdown
    Jan 15 '16 at 15:01
  • @FumbleFingers Yes indeed, that's why it's considered a marginal member of the class like dare and need. Only completely modal-like in negatives and questions. I don't know the answer to your question :) Jan 15 '16 at 15:16
  • @Araucaria: Perhaps we will be able to figure this one out once we can decide what the relationship between those two highlighted elements has to be. Jan 15 '16 at 15:26
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No, "have to" is not a modal verb, because one of the common traits between all the modals is that they can't be used with "formal" verbs like will, do, and to be. You can't say "I will must", "do I must?" and "I am musting". But you can say "I will have to", "do I have to", and, if you really need, "I'm having to". In fact, one of the reasons that "to have to" came to serve as a substitute for "must" was because it enabled people to express the idea of obligation or duty in the future tenses. (Must can somehow be used in the past). Also, if you, Egzona, are consistent, then you could ask the same question about "to be able to", which is a substitute for "can" in the future and continuous tenses.

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    Which is to say that things with near semantic equivalence may nevertheless have different grammatical restrictions.
    – Mitch
    Jan 15 '16 at 15:34

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