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"Must, have to" are all used to express obligation or the need to do something.
"Must" suggests that it is the speaker who has decided that something is necessary.
ex: I must clean the house before mum gets back. I want her to find it all neat and tidy.
"have to" suggests that somebody else has imposed the decision.
ex: Sorry, I can't come out now. I've to tidy up my room before I'm allowed out.