What would be a question to a positive sentence:
You have to eat before workout.
If you put it as:
Do I have to eat before workout?
does it mean that to have is not a modal verb in that context?
According to Wikipedia the typical modal verbs are:
can dynamic modality She can really sing. can epistemic modality That can indeed help. could dynamic modality He could swim when he was young. could epistemic modality That could happen soon. may deontic modality May I stay? may epistemic modality That may be a problem. might epistemic modality The weather might improve. must deontic modality Sam must go to school. must epistemic modality It must be hot outside. shall deontic modality You shall not pass. should deontic modality You should stop that. should epistemic modality That should be surprising. will epistemic modality She will try to lie. will deontic modality I will meet you later. would epistemic modality Nothing would accomplish that.
and have/have to is not one of them (have + past participle might be, though).
"Have to" expresses a meaning similar to must (and sometimes but not too often shall, should, will, would). modal verbs, but "have to" itself is not a modal verb.
You would never say "Do I must eat before workout" but this perfectly fine with have to.
It can be part of a modal verb phrase, such as "can have + past participle", "may have + past participle", etc. in which case it's modal.