Have to and had to

Have to is used to express some obligation, compulsion or necessity in the present or future.

I have to finish the work on time.
He has to be there by 3 o’clock.
She has to find a good job.
You have to wait for us at the railway station.
She will have to pay the dues.

Note that the expressions will have to and shall have to mean the same as must.

You will have to quit smoking if you want to get well. (= You must quit smoking if you want to get well.)

Had to

Had to is used to talk about necessity and obligation that existed in the past. Had to is the past tense form of have to.

We had to carry our own luggage.
She had to reappear for the test.
I had to consult a doctor.
At last the enemy had to accept defeat.

Used to

The expression used to is used to talk about past habits.

I used to take a nap in the afternoon. (= I no longer take a nap in the afternoon, but I used to.)
He used to go for a walk in the morning.

The expression used to can also be used to talk about existence in the past.

There used to be a kite-maker’s shop in the corner.

Used to does not have a present form. For example, the sentence ‘I use to work hard’ is wrong. The correct sentence is: I work hard.