Is " have to" a modal verb?

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?

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!

It's not an Official Modal Auxiliary Verb with Operator Superpowers, like you learned in school:

• May, might, can, could, will, would, shall, should, must; and sometimes need and dare.

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.

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.