Breakdown of Я никогда не слышал о стольких интересных категориях.
не
not
я
I
интересный
interesting
о
about (о, об, обо)
Questions & Answers about Я никогда не слышал о стольких интересных категориях.
Why does the sentence use стольких instead of столько?
The preposition о (about) requires the prepositional case. The quantifier столько (so many) changes its form in oblique cases (any case other than nominative or accusative). It declines similarly to a plural adjective, becoming стольких in the prepositional case.
I thought words like столько forced the next words into the genitive plural. Why are интересных категориях in the prepositional case?
That rule only applies when the quantifier is in the nominative or inanimate accusative case. When the quantifier is in an oblique case (like the prepositional case here, required by о), it loses its power to force the genitive. Instead, the quantifier, the adjective, and the noun all agree in the case required by the preposition.
Why is there an extra не in никогда не слышал?
Russian strictly uses double negatives. Whenever you use a negative word like никогда (never), никто (nobody), or ничего (nothing), you must still put the negative particle не right before the verb.
Why does категориях end in -иях instead of the standard prepositional plural ending -ах?