I don't agree that they are modal verbs, to the degree that they don't act like modal verbs grammatically. Modal verbs are auxiliary verbs and can take negatives and can contract. In questions, they go first. "Have" can work as an auxiliary verb ONLY in perfect verb forms (present perfect and past perfect). In these cases, you can use " 've" or "haven't/hasn't/hadn't". They can start a question: "Have you ever...?" There is always another verb in the sentence, and that other verb is the main action. "Have" can also be a main verb, of course. In these cases, you use "don't/doesn't have" and "Do you have...?" "Have to" as an obligation works as a main verb. "Don't/doesn't have to", "do I have to...?" and so on. "Haven't to" doesn't exist. Nor does "Have you to...?" Beware of this error.

'Have' is used as a normal verb by itself (tener) or as an auxiliary verb (haber) in perfect tenses with the past participle. 'Have to' is used as a modal verb and means the same as 'must'. (deber)

Hello! If you say " I have a car" it's just stating a fact: you are the owner of a car. If you say " I have to have a car" you're saying something different: you don't have a car but you MUST have one. So "have" would be the equivalent of "tener" in Spanish (tengo un coche/carro) and "have to" would be the equivalent of "tengo que (tener)" which expresses an obligation or a need (aunque en castellano suene un poquillo raro, se traduciria mejor por "necesito") ;) I hope this helps you! C.