Our previous lesson explored the use of the verbs haber and deber to express obligation, requirement, or necessity. Let's now see how we can used them to express possibility or supposition instead.

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We learned that the construction haber + de + infinitive is used to express obligation or necessity. The following example, however, shows that it's also possible to use it to express possibility:
 

Si mi vida ha de continuar... será por ti

If my life should continue... it will be for you

Captions 7-10, Belanova - Por ti

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Of course, as you may have noticed, the use of the conditional si (if) plays an important role here. By using it, a sentence that would otherwise express a certain necessity, like mi vida ha de continuar (my life should continue), is transformed into one that expresses possibility. We can also use other words or phrases besides the conditional, for example, words typically used to express possibility, supposition, conjecture, or doubt. Here is an example using tal vez (maybe):
 

Tal vez ha de haber sido... un intento de mi parte.

Maybe it must have been... an attempt on my part.

Captions 24-25, Arturo Vega - Entrevista

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However, sometimes we have to figure it out by the context. See for example this fragment of the song Marciano (Martian) by Mexican band Molotov:

No es el cuerpo marrano que solía tener, ni la cara, mi reina, que tú has de querer.
It's not the fat body I used to have, nor the face, my queen, that you probably want.

In fact, English is not unfamiliar with the use  of "should" and "must" to express probability in sentences that rely on context for interpretation. Here are two examples:

Ha de tener hambre. | She must be hungry.
Has de querer que te de dinero. | You must want me to give you money.

You must also know that the expression ha de ser by itself means "perhaps" or "maybe":

Creo que ella es la ladrona. -Sí. Ha de ser.
I believe she is the thief. -Yes. Perhaps.

More interesting yet is the use of the verb deber (which is also a noun meaning "duty") to express possibility in Spanish. In our previous lesson you learned that deber + infinitive is used to express obligation. You also learned that it's grammatically incorrect, though common, to use deber + de + infinitive for the same purposes: you mustn't say debes de hacer la tarea, you must say debes hacer la tarea (you must do your homework). This is because deber + de + infinitive is reserved in Spanish to express possibility. Here are some examples:
 

Ah el... Este debe de ser el contrato.

Oh the... This must be the contract.

Caption 15, Muñeca Brava - 9 Engaños

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So, in the case of the verb deber, Spanish uses two slightly different constructions: debe + de + infinitive for possibility, and debe + infinitive for obligation. Compare the previous example with the following:

Yo insisto en que éste debe ser el contrato.
I insist that this must be the contract.

Here is another example where debe de is correctly used to express possibility: 
 

Así que creo que le debe de haber sido muy difícil.

So I believe that it must have been very hard for her.

Caption 47, Biografía - Natalia Oreiro

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But, as we mentioned before, the use of debe + de + infinitive instead of debe + infinitive to express obligation is a very common mistake. We should avoid doing it, especially in written or formal Spanish. Not that it's a big deal, but below is one example (and you can find many more similar cases by searching our catalog): 
 

Y esto lo debe de pagar* el cliente porque es de otro siniestro.

And this must be paid by the customer because it's from another accident.

Captions 17-18, 75 minutos - Gangas para ricos

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*Y esto lo debe pagar el cliente is the correct expression.