Thanks so much for your answer.

You could also look at the these as a percentage of certainty. 98-100% sure use "must" or "have to" 70-85% sure use "should" 50% sure use "may" or "might"

They are all very close in meaning. However, the word "should' is conditional and "must" is imparative. If you say, "Your should probably see a doctor," it means it would be good to see a doctor, but it's not completely necessary. If you say, "You must see a doctor," it really means it's very important that you go see a doctor quickly. "Have to" is a lot like "must." "You have to do it" and "You must do it" mean the same thing. It means you really have no choice except to do it. "You should do it," means it would be good if you did it, but it's not too important.