Because I kept getting this wrong, I asked Chatgpt: Can one use deber conjugated in El Pretérito Perfecto, followed by an infinitive to express in Spanish that something must have happened, as when making an assumption or wondering what must have happened? The response was: No—not if your goal is to express an assumption ("must have happened").
The pretérito perfecto of deber (ha debido) normally means "has had to" (obligation), not "must have" (probability).
Hola Ronald M.
Thanks for your question! In this case, ChatGPT's answer is an oversimplification.
When expressing a deduction about a past event, deber can indeed be used in the pretérito perfecto followed by an infinitive, for example:
In everyday Spanish, especially in speech, many speakers also omit the de and simply say ha debido pasar, although traditional grammar prefers deber de when expressing probability.
On the other hand, ha debido + infinitive can also express a past obligation ("has had to..."), so the intended meaning depends on the context.
So, while ChatGPT is right that ha debido often expresses obligation, it is not correct to say it cannot express probability. Both uses exist, and context determines the intended meaning.
Un saludo
Silvia
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