We use have to:

• to show it is necessary to do something.

EXAMPLE: I have to leave now as they are waiting for me.

• when circumstances dictate that we do something.

EXAMPLE: He has to see the doctor every month to treat his skin disorder.

• for the present or future.

EXAMPLE: All of us have to attend a meeting now.
EXAMPLE: We have to work late tomorrow.

• with the past tense.

EXAMPLE: We had to do the training in order to be eligible.

• in place of have got to. There is no difference in meaning.

EXAMPLE: I have to/have got to give a talk tomorrow.