The difference between "¿Qué es ..." and "¿Cuál es ..."

Question

I am in the middle of doing your writing exercise "Castile comes from 'Castle'" > https://spanish.kwiziq.com/my-languages/spanish/exercises/take/667/6083595?page=6 ... and incorrectly used "¿Qué ... ?" to translate "What is its origin?" ... I have to confess that the difference between "¿Qué es ... ?" and  "¿Cuál es ... ?" was something which I had not yet grasped properly !
I consulted my grammar book, which explains that "¿Qué es ... ?" really means "What kind of thing?", i.e., inquiring about the definition of something's nature.... > which is of course what you say in your reply [below] to R.Z., [Thank you Inma !] - and which is supplemented by other comments from Kwiziq students.
It might be worth including this distinction in the grammar-notes which accompany the above-mentioned writing exercise.

Silvia Piriz · Accepted Answer

Hola David M.
Thanks for your thoughtful comment and for sharing your process so clearly!
You're absolutely right about the subtle but important difference between ¿Qué es...? and ¿Cuál es...?. It's a tricky point even for advanced learners and your summary captures it well.
We've now added a hint to the exercise to help clarify this distinction in context, so future learners can benefit from it while completing the task.
Thanks again for your suggestion, it's always helpful to have insights from engaged learners like you!
Hasta pronto
Silvia

David McNaughton · Answer

Thank you Silvia ... The topic does indeed need careful attention, because [in European Spanish, at least] the rule seems to change when the word is used adjectivally [i.e., directly before a noun] e.g. -

> "Which girls are coming tonight?" = "¿Qué chicas vienen esta noche? ... [Not: "¿Cuáles chicas ... ?"