Karen, we use "should" in English mainly as a suggestion. "You should study more. He should work harder." On the other hand, we use "must" not as a suggestion, but as a requirement. "You must study more if you want to get better grades. He must work harder if he wants to get a promotion".

If you search for 'modal verbs' you will find a lot of information on this. In general: MUST is "yo debo" or "yo tengo que..." and SHOULD is "yo debería". There isn't much difference in Spanish but in English the difference is that 'must' has a lot more force, whereas 'should' is a friendly suggestion or idea.

Uso "should" cuando quiero sugerir algo, por ejemplo, "You should listen to Michael Jackson's Thriller album." (Tambien lo uso cuando necesito hacer algo...como...I should go to the store before it closes.) Uso "must" cuando estoy hablando de algo que es obligatorio. Una doctora podria decir "You must lose weight if you want to avoid having another heart attack." Un jefe puede decir al empleado "If you want to work here, you have to sell more products or I will have to fire you." (Si quieres trabajar aqui, es necesario que vendas mas productos o te voy a despedir.") Ojala que esto le ayude.

it is different in British english and American english. USA: Must = debes (it's necessary) "You must drink water to survive" UK: Must = deberias (or a suggestion) "you must go to the palace while you're in London" Should is the same in both