A matéria é difícil, mas as perguntas são fáceis
Why would the (permanent verb) ‘‘ser’’ be used here when clearly these are not permanent conditions.
Olá, @marlize! The temporary vs. permanent thing is a rule of thumb that won’t apply in every context. Still, if we choose to look at this sentence through that lens, I don’t see an incompatibility with the verb ser. Both the subject and the questions are what they are and won’t change, and the speaker is sharing their subjective perception of them, which is also presumably established and not expected to change in short notice (even though it could). So, we could think of this as being ‘permanent’, or stable.
As an alternative, we can also think of it as intrinsic vs. non-intrinsic. In the speaker’s perspective, these questions are intrinsically easy by the way they’re made, hence ‘ser’. Another quality that we usually consider intrinsic in regard to questions/any sentence is coherence - so, for example, if the questions seemed incoherent due to poor phrasing, we would use the verb estar to describe this, because it goes against what’s expected of a sentence’s nature.
- As perguntas são fáceis.
- As perguntas estão mal formuladas.
This is way more overthinking than any native speaker would do, but I think it might help you make more sense out of these verb choices!
Thank you. Thais sheds a little more light on the ser estar conundrum.