Selon la légende, des spectres hantent les salles. J'ignore ce qu'il y a de vrai là-dedans, mais...
In this sentence, there are two things I cannot understand:
Why the need to say "j'ignore ce que {I don’t know what}" instead of "j'ignore si {I don’t know if}"?
I was under the impression that the phrase "il y a du vrai là-dedans" is a fixed expression with the meaning of "there’s some truth in that". So why "de vrai" rather than "du vrai"?