"Should" means that "it is recommended", "it is better," or "it would have been better" etc. I should go for a walk. (This is something I ought to do; it would be better if I did, but I am not obligated to go for a walk.) You should do your homework. (again, doesn't imply obligation, though slightly stronger here) ------------ "Must" implies a strong obligation. If you "must" do something, then you don't have any other choice. It must be done. "Have to" is slightly less formal. It can only be used with obligation. "I have to pick my friend up at 5". But the implication is a lot looser, and it can be used when there is no obligation. For example: "I have to go home because my favorite TV show starts in 30 minutes". There is no obligation for you to go home, but the only way for you to watch your TV show is for you to go home, so in that sense you "have to". "Must" wouldn't work here, unless you have an obligation to watch this show. ------------------- Must can only be used to show certainty in an opinion. For example: "The lights aren't turning on. The lightbulb must be broken." This doesn't mean it is obligated, just that that is the most likely scenario. "Tim isn't here. He must have gone out." Again, no obligation, but given the situation, this is the most likely result, so it is used to express certainty in my opinion. It is the way "things must be".

simply : should for suggestions(maybe opinion). must for express a strong point of view. have to for saying that something is obligatory.