i wanna ask a question.please make me clear
which one is stronger; must or have to.
Neither. They are synonyms.
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I think there is a little difference at least in negative forms of "must" and "have to".

You can say:

You must not do this. ( = It is necessary/obligatory that you don't do this ).
You don't have to do this. ( = It is not necessary/obligatory that you do this ).

Correct me if I err.
There is of course a big difference between 'Have to' and 'Must'.
You might say that both of them express a kind of obligation, but the difference lies in the source of obligation.
'Have to' is usually used toexpress an obligation from the outside, i.e external obligation - authority, power, parents, teacher. While 'Must' expresses an internal one, i.e one that is coming from the inside.

Pay attention!

I have to work eight hours a day. (this means I cannot work less or I'll be fires by my boss for example)

I must work eight hours. (this means I need to work eight hours because I think this is good for me)

of course sometimes Must is used to express external obligation but it sounds bossy, and therefore people avoid it unless they want a kind of overstatement.
You must watch that movie. (Of course this is not bossy, but means that I advise you to watch that movie because it is really great)