Have to is often grouped with modal auxiliary verbs for convenience, but in fact it is not a modal verb. It is not even an auxiliary verb. In the have to structure, "have" is a main verb. The structure is:
subject + auxiliary verb + have + infinitive (with to)
Look at these examples in the simple tense:
main verb have
infinitive (with to)
Use of Have to
In general, have to expresses impersonal obligation. The subject of have to is obliged or forced to act by a separate, external power (for example, the Law or school rules). Have to is objective. Look at these examples:
In France, you have to drive on the right.
In England, most schoolchildren have to wear a uniform.
John has to wear a tie at work.
In each of the above cases, the obligation is not the subject's opinion or idea. The obligation is imposed from outside.
We can use have to in all tenses, and also with modal auxiliaries. We conjugate it just like any other main verb. Here are some examples:
main verb have
"To Be To" as a modal verb is used in two tense forms - Present and Past Simple.
Obligation arising out of an arrangement or plan/in statements and questions/.
We are to complete this work by tomorrow.When is the wedding to be?
A strict order or instruction, given by the speaker or any other authority.
You are to do it exactly you were told.you are to stay here until I return!
They are not to be trusted!Nothing was to be done under the circumstances.
Perfect Infinitive is used to emphasize that the action did not take place.
I was to have come.I was to have graduated in June but failed.
You are not to do that!You are not to tell anything about it!
In the expressions:
What am I to do?What is to become of me?Where am I to go?
A purpose or a plan.
A knife is to cut with.This prize was to honor him for his success.