Do you know the difference between "deber" and "deber de"? This article explains both uses and gives you some examples, so you can understand it.

The first diference, at a glance, is that one of these forms includes the preposition "de" ("of" ). But the main difference is in meaning:

• Deber (+ infinitive). The Spanish verb "deber" has several meanings: to owe, to have to, must. In order to explain the difference between "deber" and "deber de", we will use "to have to" / "must" meanings. So, according to this, "deber"+ infinitive implies the idea of obligation.
  
  (Note "deber" is also a masculine noun: "el deber" ("the duty" or "the homework"), but we are interested now in "deber" as a verb.)
  
  For example:
  Debo ir a la oficina. (I have to go to the office.)
  Debes leer ese libro. (You have to read that book.)
  
  As you can see, the form "deber" (conjugated) in both examples implies the idea of obligation.



• Deber de (+ infinitive). We use the verb "deber" followed by the proposition "de" to express the idea of doubt or uncertainty, that is, when we are not sure about something. It can be translated as "it is possible that…" ("Es posible que…")
  
  For example:
  Debe de ser tarde. (It must be late.)
  Juan debe de estar diciendo la verdad. (Juan must be telling the truth.)

In both cases (Deber / Deber de), we will conjugate the verb "deber", according to the tense, mood and person we may need. Some examples:

Debo ir a la escuela. (I must go to school.) - Present tense, indicative, first person singular (Yo debo)Ana debería de llegar en una hora. (Ana should be arriving in one hour.) - Conditional simple, indicative, third person singular.
Note this difference is not always known by native Spanish speakers! It is quite common to hear someone (native Spanish speaker) saying: "Debe ser tarde", when in fact, what he/she means is : "It must be late", that is, expressing the idea of doubt or uncertainty.

So, that person should be saying "Debe de ser tarde", because "Debe ser tarde" implies obligation and "Debe de ser tarde" implies doubt, uncertainty.

In short:

- Deber + infinitive → Obligation
- Deber de + infinitive → Doubt, uncertainty

I have created a quiz ao you can practice and learn to differentiate and use "deber" and "deber de".
You can find it here: Quiz "deber" vs. "deber de"


Are you looking for more information about Spanish verbs? IF so, you may find this book intereseting: 501 Spanish Verbs with CD-ROM and Audio CD (501 Verb Series)