In what situation should I use 「have to」 and「must」 properly?
「Have to」 is used for the opinion from the outside and「must」 has been heard use to the thing that I thought of.
Please tell me.
Anonymous「Have to」 is used for the opinion from the outside and「must」 has been heard use to the thing that I thought of
You have a rough idea of the official difference: 'have to' is used of an external command and 'must' is used of a command or feeling of necessity from within oneself.

Keep in mind, however, that (1) some native speakers do not differentiate, but use one or the other indifferently or by personal preference, (2) many native speakers feel that 'must' is merely more formal, and (3) many native speakers use one over the other for social convenience or manipulation (i.e, use one when the true situation is in the other).

In addition, there is no past form for 'must' (in the sense of command), so we must use 'had to' synonymously in that case:

I have to attend class today. (a school requirement)
I must study English this afternoon. (internal compulsion)
I had to attend class today. (a school requirement)
I had to study English yesterday afternoon. (internal compulsion)
Thank you. I was good very much.