請問一下 it 在這裡的用法?

1. that 是關係代詞 (relative pronoun) 代替 question.
e.g. It (the mirror) was asked the question by somenone. (passive voice for the verb "ask")
Active voice: Someone asked the mirror the question. (mirror - indirect object, question - direct object)

2. it 的用途 沒有it可以嗎? 沒了意思會改變嗎?
"it:" is a pronoun representing "the mirror", the subject of the relative clause and "the question" is the object of the relative clause. Without "it", "the question" becomes the subject of the relative clause. The grammar is still correct but the meaning is slightly different. (= 所有被問的問題，包括問鏡子，問王后，問任何人。)

3. question that it was asked
解作: 魔鏡被問的問題

1.本句裡的第一個關係代名詞that，代表前面的名詞the magic mirror。這裡屬限定性質的修飾語，magic mirror應加定冠詞，或首字母大寫，從普通名詞變成專有名詞。that和and前面的逗號均應刪去。
2.第二個關係代名詞that，代表前面的名詞question。
3.代名詞it代表前面的名詞the magic mirror。... any question that it was asked =鏡子所被問到的任何問題。it是這個形容詞子句的主詞，不可省略。that是帶出形容詞子句的關係代名詞，建立與先行詞question的修飾語關係。