Hello, I'm slightly confused with the following:
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.
So you are saying that "de" is truly optional and has the same meaning whether "de" is used or not?
Yes, you can use deber or deber de if what you imply is an "assumption" - here is a lesson on this precise topic.
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