‘Now’ is ‘Xian zai’ in Mandarin.

You’re correct. You can say ‘ni xian zai zuo shen me’. And you can also put time words at the beginning of a sentence: ‘xian zai ni zuo shen me’. Both are correct.

But you cannot put time words at the end of a sentence, like ‘ni zuo shen me xian zai’, though it makes sense. It’a difference between English and Mandarin.