You know what a fairy is

这里what引导的是一个宾语从句，know 是谓语, what a fairy is是宾语,用了陈述句语序。所以is在后面。
whatis your name? (特殊问句)
Can you tell what your name is? (宾语从句)

附: 宾语从句讲解

宾语从句（一） 宾语从句在复合句中作主句的宾语。宾语从句常由that引导，在口语中常省略。 当主句中谓语动词是现在或将来时态时，从句中谓语动词不受主句中谓语动词时态的 影响，按需要可以使用任何时态。如： She says (that) she works from Monday to Friday. 她说她从周一至周五上班。（从句是一般现在时） She says (that) she will leave a message on his desk. 她说她要在他桌子上留个便条。（从句是一般将来时） She says (that) she has never been to Mount Emei. 她说她从来没有去过峨眉山。（从句是现在完成时） 当主句谓语是过去时态，从句中的时态一般用表示过去的某种时态。如： He said there were no classes yesterday afternoon. 他说昨天下午没有课。（从句是一般过去时） He said (that) he was going to take care of the baby. 他说他会去照看这个婴儿。（从句是过去将来时） He said (that) they were having a meeting at that time. 他说他们那时正在开会。（从句是过去进行时） *当主句谓语是过去时态，而宾语从句叙述某一客观真理（事实）时， 宾语从句的时态则用一般现在时。如： The teacher told us(that) nothing is difficult if you put your heart into it. 老师告诉我们世上无难事只怕有心人。 She said (that) her father is twenty-eight years older than her. 她说她父亲比她大二十八岁。 He said that li