shall / must / should / ought to / have to

have to

I have to do something = it is necessary for me to do it, I am obliged to do it

must and have to

You can use must or have to when you say what you think is necessary, when you give your opinion:

It’s a fantastic film. You must see it. or You have to see it.

When you are not giving your personal opinion, use have to (not must). Compare:

Jane won’t be at work this afternoon. She has to go to the doctor.
(this is not my personal opinion – it is a fact)

Jane isn’t well. She doesn’t want to go to the doctor, but I told her she must go.
(this is my personal opinion)