• "You should" is always translated as "deberías".
• It is used to give advice (in affirmative or negative):
  
  You should go to the doctor if you have a temperature.
  You shouldn't go out if you're feeling dizzy.

• "You must" is translated as "debes".
• It is used to express an obligation (affirmative) or a prohibition (negative):
  
  You must treat others as you would like them to treat you.
  You mustn't smoke in a public place since they passed the anti-tobacco law.
  
  
• It is also used for a very insistent piece of advice:
  
  You must see a doctor immediately if you feel a pain in your heart.
  

• "You have to" means "tienes que"
• In the affirmative, its meaning is very similar to that of "must", but it is used to express a necessity rather than an obligation:
  
  You have to study a lot to become a doctor.
  You have to break some eggs to make an omelette.
  
  
• However, in the negative it is used to express absence of obligation only:
  
  You don't have to finish this now if you don't want to.
  
  
• Contrast MUSTN'T and DON'T HAVE TO in these examples:
  
  You don't have to copy the instructions. (= It is not necessary to copy the instructions.)
  You mustn't copy the instructions, otherwise you will not have enough time to finish the exercise. (= It is a bad idea to copy the instructions.)
  
  

JJCC