To express in Spanish that something must have happened, as when making an assumption or wondering what must have happened, we can use the verb deber in two different ways:
Deber (de) in El Pretérito Perfecto
We can use deber conjugated in El Pretérito Perfecto, followed by an infinitive. For example:
Here we are not talking about obligation, which is one of the uses of the modal verb deber, but instead are wondering/assuming what must have happened. The reason to make such an assumption is very often expressed by a fact.
"Cristina ha debido de ser una buena profesora." (assumption)
"Sus estudiantes le han comprado flores." (fact)
Deber (de) in El Presente + haber + participio
The same thing can be expressed using this structure with deber. Have a look at the examples again:
As you can see from the previous examples, when we use deber with this nuance of assumption/wondering, we can optionally use the preposition "de" after deber.
When the main verb of the sentence is a pronominal verb (uses a reflexive pronoun) you can place the pronoun either in front of the main verbal structure or attached to haber. For example:
This would be incorrect:
"Su avión debe haber retrasado se.
"Su avión ha debido se retrasar.
"Su avión ha se debido retrasar.
Note that when we use the modal verb deber conjugated in:
- El Pretérito Indefinido + infinitive
- El Pretérito Indefinido + perfect infinitive
the meaning is generally different. It would normally be translated as "should have done something" instead.
See also Spanish modal verb Deber versus Deber de (obligation and assumption) and Spanish modal verb deber in the conditional simple + haber + participle = should have
For a different way to make suppositions about the past see also Using the Spanish future perfect to talk about the past (El Futuro Perfecto)
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