Which of these expressions is correct? "Debe ser" vs "Debe de ser" [duplicate]

Question

I have always had this doubt.

Which of these expressions is properly used and when to use each of them?

Debe haber sido un accidente

Debe de haber sido un accidente (is this a mistake?)

Community · Accepted Answer · 2020-06-17 09:53:29Z

In your example both forms are correct.

According to the RAE about the uses of deber:

Debo cumplir con mi misión _{I must accomplish my mission}.

In this case (denoting an obligation) is an error to introduce the preposition "de".

Debo de cumplir con mi misión (WRONG!!)

No hay luz en su casa. Deben de haber salido _{There is no light at their place. They must have gone out}

Debe de haber sido un accidente.

When expressing a supposition, you can use both "deber de + infinitive" or just "deber + infinitive".

Marianita, su hija, debe tener unos veinte años _{Marianita, their daughter, must be twenty years old} (expresses a guess, not an obligation)

Since you example denotes a supposition, both forms provided are OK.

The answer is correct, but "debe + de" would be the construction of choice in this case. Originally, both constructions had different meanings, as Diego explains; but, through time, the use of deber, without preposition, was extended to also cover the meaning of deber de, so the RAE accepted it a few years ago. But using deber + de removes the ambiguity from the sentence. Debe haber sido un accidente might also mean that, once the evidence has been analysed, the only logical conclusion is that this was an accident. — Gorpik, Feb 16, 2015 at 8:57

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