When I was first learning Japanese, I learned that the explanatory form, -んです is always constructed by conjugating the verb in short form and adding -んです, and that the です part is never conjugated. But in more than one instance, I have seen -んじゃない used for the explanatory form. For example:
おはしの使い方を知らないんです。
as opposed to:
おはしの使い方を知るんじゃないです。
What does it mean when the -です is conjugated rather than the main verb?