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Is "have to" a modal verb? tells us, in two conflicting answers, that it's either a modal verb or it's not a modal verb.

Then I just realized that there are never any definitions provided for what a modal is, only examples of modal verbs, or examples of what they do. It's an all familiar thing: "modals are used to express obligation, necessity, permission . . . ".

To narrow down and clarify the question,

  • What properties should an item possess to qualify as a modal?
  • Are modals and modal verbs different things? In other words, are there modals that are not verbs?
  • Are there modal verbs that aren't auxiliary verbs?

I realize there might be conflicting definitions, but there are areas most experts would agree upon, and for the shadier parts of the definition, only one definition would suffice.

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    It's too big a big topic for a short answer, but just to respond to your first question: no, "have to" is not a modal verb, though it does express obligation or necessity similar to "must". In fact, for most people it is not even an auxiliary verb, but a lexical one, cf. lexical "Do I have to do my homework"? with the much less likely auxiliary "Have I to do my homework"? Incidentally, the verb is just "have"; "to" is a separate subordinator, part of the clause that is complement to "have". – BillJ Apr 17 '17 at 18:34
  • M.A.R, in that question you linked to, there is no answer that actually tell it's a modal verb. All answers there agree that it's not a modal verb. – Man_From_India Apr 17 '17 at 23:45
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In modern grammar, modal verbs are a grammatical category of verb. They are a subcategory of the auxiliary verbs. There are nine modal auxiliary verbs in English: CAN, COULD, SHALL, SHOULD, WILL, WOULD, MAY, MIGHT and MUST.

Modal verbs are different from other auxiliary verbs like BE HAVE and DO and also from other verbs in general.

A: Tense:

Firstly, modal verbs always have tense. They are always present tense or past tense:

  • PRESENT TENSE: can, shall, will, may, must.

  • PAST TENSE: could, should, would, might.

B: They don't change:

Secondly, modal verbs never change. They always stay the same. They have no third person S in the present tense:

  • walks, is, has, does, cans

They have no plain form (no 'bare infinitive'). And for this reason we don't see them in to-infinitives. A to -infinitive is just the word to followed by the plain form:

  • to walk, to be, to have, to do, to may

They have no -ing form (no present participle):

  • walking, being, having, doing, musting

They have no past participle (no 3rd form):

  • walked, been, had, done, musted

C: Only one modal

Other auxiliary verbs can occur together:

  • I have been swimming.

However, we never find two modal auxiliary verbs together:

  • *When I finish college, I will can speak English very well (ungrammatical)
  • When I finish college, I will be able to speak English very well.

D: Always the first verb:

A modal verb is always the first verb in the verb phrase:

  • *She ____ have been studying might English. (ungrammatical)
  • She might have been studying English.

More information

Modal verbs often tell us about modality. This means they tell us about possibilities and necessities, or rules and obligations. However, other verbs and other types of word also have meanings relating to modality. The important fact about modal verbs is that they have a certain grammar. They are a grammatical type of word.

Some verbs sometimes have a similar grammar to modal verbs. For example the verbs OUGHT, DARE and NEED. Some people call these semi-modals.

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What is a modal, really?

A miserable pile of words that together form a verb phrase.

Seriously, though, first look at how English can be considered to have "verb forms" that consist of two or more words, containing be and have as a "helping" or auxillary verb. This occurs with perfect tenses, passive forms, and progressive/continuous forms.

I have walked to the store.

I am walking to the store.

The paint was applied on the car by the painter.

Modals kinda-sorta follow the same principle and are additional "option" words that further modify the verb and make it conditional and/or add other shades of meaning relating to predicting the future and/or expressing indirect requests/commands.

From Wikipedia, here are the modals and words that can sometimes be modals:

The principal English modal verbs are can, could, may, might, must, shall, should, will and would. Certain other verbs are sometimes, but not always, classed as modals; these include ought, had better, and (in certain uses) dare and need. Verbs which share some but not all of the characteristics of the principal modals are sometimes called "semimodals".

To turn to walk, for example, into present perfect form, you would say I have walked. Have changes according to subject, not walk anymore. E.g. I have walked, he has walked, he/she/it has walked.

The modals operate the same way but don't change form according to subject. E.g. I may walk, he may walk, he/she/it may walk. So it's part of a verb, but modals never occur without a verb. If they do, part of the verb is being conversationally elided. E.g. "I knew he would." = "I knew he would [do that]."

Modals + to have and to be in the perfect/progressive forms exist. E.g. I may have walked, she was walking but not in the passive form unless you use have - e.g. The paint was applied by the painter -> The paint may have been applied by the painter.

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