Have to and be+going to difference

Question

As far as I understand, both "have to" and "be + going to" are used in future action.

So, for example I have an exam in 2 hours, which one should I say?

1. I think I have to study now because I have an exam in 2 hours.
2. I think I'm going to study now because I have an exam in 2 hours.

What's the main difference? Are those interchangeable?

I read a book and it said have to expresses the idea that something is necessary and be + going to is used when I want to do something. In my case, I want to study, so, aren't they have the same meaning? By the way I've tried to look for possible duplicate but I couldn't find any. Hope you can help me to understand.

Answer 1

1. "I have to"
2. "I'm going to"

These two examples have different meanings, as you have checked too.

As you want to study though you do not need to, the second example is better.

https://dictionary.cambridge.org/dictionary/english/have-to

have to auxiliary verb

(used with the infinitive form of another verb) to need to or be forced to; must:

I have to go to Vermont tomorrow.

https://dictionary.cambridge.org/grammar/british-grammar/future-be-going-to-i-am-going-to-work

Future: be going to

Intentions

We use be going to to talk about future plans and intentions. Usually the decision about the future plans has already been made:

She’s going to be a professional dancer when she grows up.