have to for objective obligation

We often use have to to say that something is obligatory, for example:

Note that we can use the have to expression in all tenses, for example: he has to, he had to, he has had to, he will have to

Structure of have to

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 basic structure for have to is:

subject + auxiliary verb + have + to-infinitive

Look at these examples in the Present Simple tense:

subject auxiliary verb main verb
have
to-infinitive
+ She has to work.  
- I do not have to see the doctor.
? Did you have to go to school?

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 each of the above cases, the obligation is not the subject's opinion or idea. The obligation comes 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:

  subject auxiliary verb main verb
have
to-infinitive  
Past Simple I   had to work yesterday.
Present Simple I   have to work today.
Future Simple I will have to work tomorrow.
Present Continuous She is having to wait.  
Present Perfect We have had to change the time.
modal may They may have to do it again.