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".
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.
- It is a false assumption to say that "have to" is a modal verb of obligation.
- 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)
Best Answer
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:
cansThey 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 mayThey have no -ing form (no present participle):
mustingThey have no past participle (no 3rd form):
mustedC: Only one modal
Other auxiliary verbs can occur together:
However, we never find two modal auxiliary verbs together:
D: Always the first verb:
A modal verb is always the first verb in the verb phrase:
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.