La Diferencia Entre "Deber" y "Deber de"

One of the most common mistakes in the usage of Spanish grammar, for not just Spanish language learners but for native speakers as well, is the mistake of using deber instead of deber de, or vice versa.  Hopefully this brief lesson sheds some light on the whole mess for you.

I'm going to go ahead and assume that you have your verb charts handy and/or memorized so you are familiar with the conjugation of this verb (which is regular -er in all tenses and moods).

The difference is that deber + infinitive indicates obligation/necessity to do something, whereas deber de + infinitive indicates a probability of something occurring with a sense of definiteness ("must be", "probably are").  Let's look at some examples.  First, deber + infinitive (which is very similar to tener que + infinitive (to have to)) is constructed like this:

Debo lavar mi carro.  I should/need to/have to/must wash my car.  

Emilia debe pasar la aspiradora.  Emilia should/needs to/has to/must vacuum.  

Tú debes prepararte para la prueba.  You should/need to/have to/must get prepared for the quiz.

Nosotros debemos apagar las luces.  We should/need to/have to/must turn out the lights.

Vosotros debéis manejar a Toluca.  You all should/need to/have to/must drive to Toluca.  

Los empleados deben llegar a tiempo.  The employees should/need to/have to/must arrive on time.  

You'll notice in the aforementioned examples that each use of deber could very easily be replaced by the more common tener que.  The use of deber is slightly more aligned with the meaning of "should" or "ought to", but it still implies a sense of duty or obligation.

Let's now switch gears and check out deber de + infinitive.  This verb structure indicates a sense of probability of something occuring.  While it seems a minor difference with a simple addition of de, it is still a very common mistake for intermediate to advanced Spanish learners, as well as for native speakers alike.  Although the difference seems miniscule, it completely changes the meaning of the sentence.

Debo de saber la diferencia entre los verbos.  I probably/must know the difference between the verbs.

Debes de estar confundido.  You probably are/must be confused.

Juanito debe de estar trabajando.  Juanito is probably/must be working.  

Nosotras debemos de estar locas.  We probably are/must be crazy.  

Tu novia y tú debéis de estar enamorados.  You and your girlfriend probably are/must be in love. 

Ellos deben de estar dormidos.  They probably are/must be asleep.  

So in essence, if we take the verb conocer and apply it to both constructions, we can immediately see the difference in meaning of both utilizations:

Debemos conocer Barcelona.  We should/ought to/must/have to be familiar with Barcelona.

Debemos de conocer Barcelona.  We probably are/must be familiar with Barcelona.  

Ojalá que esta lección te ayudara...

Profesor Joel