The verb "have to"
Have to is not a modal verb. It changes like an ordinary verb (e.g., "want to do something").
Mike has to leave.
Mike doesn't have to leave.
Does Mike have to leave?
Mike had to leave.
Did Mike have to leave?
etc.
What does "have to" mean?
have to = to be obliged to do that, it is necessary to do that
not have to = it is not necessary to do that, there is no need to do that
Examples
My phone fell down and broke down. I had to buy a new phone. = I didn't have any options so I bought a new phone.
I have an early flight so I have to go to bed now. = It is necessary for me to go to bed now.
We didn't have money for the taxi so we had to walk home. = We didn't have any options so we walked.
It is the same contract as last year. You don't have to read it through again. = It is not necessary to read it through again.
You don't have to sit here and wait. I'll call you when Richard is back. = It is not necessary to sit here and wait, you can go.