Must

• Modal verb of obligation.
• The speaker thinks it is necessary.
• La obligación es PERSONAL. Obligaciones externas en contextos formales (la ley o las reglas).
• Negative: prohibition. Cuando NO es necesario ejecutar una acción.

He musn't call her at work.

S + MV + V. infinitive + C

Have to

• Modal verb of obligation.
• External obligation: cuando para alguien externo es una obligación.
• No obligation: you can do something but you don't need to do it.
• Give an advice.

You have to cut down on sugar.

S + MV + V. infinitive + C.

Should

• Modal verb of obligation.
• Giving advice: to say what is or isn’t the right thing to do.
• When something is possible (probable).
• Para mostrar obligación o dar una opinión.

You should stop eating fast food.