The verb deber is quite common and can be used to express obligation or likelihood.
Expressing obligation: When used in this way, deber is frequently similar in meaning to the English "must," "should" or "ought":
- Debes estudiar tus lecciones. You ought to study your lessons.
- ¿Qué carro debo comprar? Which car should I buy?
- Alguien deberá hablar de todo esto. Someone will have to talk about all this.
- Son las palabras que no debiste decir. You shouldn't have said those words.
- No debe dormirse después de comer. He shouldn't go to sleep after eating.
The tone of deber to express obligation can be softened by using the conditional form instead of the present tense, although the difference isn't always translatable. When telling someone what he or she should be doing, using the conditional can come across as more polite:
- No deberías abrir un blog si vas a hablar de temas personales. You shouldn't start a blog if you're going to talk about personal subjects.
- La fuerza aerea debería comprar mejores aviones de combate. The air force should buy better combat planes.
When used with a noun direct object, deber can usually be translated as "owe."
- No me debes nada. You don't owe me anything.
- El gobierno le debe más de $3 millones a mi madre. The government owes my mother more than $3 million.
Expressing strong probability: The phrase debes de is used to refer to a strong likelihood. In such cases, it is often the equivalent of the English "must" when "must" isn't used to express obligation:
- No debiste de firmar nada. You must not have signed anything.
- Debo de ser anormal. I must be abnormal.
- Deben de estar en buena condición. In all likelihood, they are in good condition.
It is very common in some areas, especially in speech, to drop the de in sentences such as those above. However, out of context, doing so would make the sentences ambiguous. Thus while "debías de verme" can mean only "you must have seen me," "debías verme" can mean either "you must have seen me" or "you ought to have seen me." In that case, you would need to know the context to understand what was meant.
On occasion, in some regions, you may hear deber de to express obligation. However, this usage is frowned upon by grammarians and probably should not be imitated if you are learning the language.