どうして vs. なぜ: what is the difference?
In the sentence "Why is the test difficult?"
そのテストは なぜ むずかしいんですか
it is wrong, apparently, to use どうして instead of なぜ. Can someone explain why? And how would I have to change the sentence if I wanted to use どうして instead?
It is not wrong. どうして and なぜ have the same meaning. なぜ is just more formal than どうして. So, you'd use どうして with friends and family, and なぜ in formal situations. I recommend reporting it as a correct answer if you get something similar to this.
In addition to なぜ being more formal than どうして, there is a subtle difference in terms of the origin of the two: どうして＝どう（How) +して（do), for which the closest English equivalent is (How come/ how does this happen, etc.), while なぜ is plain (Why). While we are at it, let's add another word with the same usage : なんで ＝何（what) +で（with, by, via), translating literally to (by what), and indicating (what's is the reason for/origin of ). Based on the breakdown of these expressions, you can probably guess when to use which. Finally, and higher on the formality ladder, there is なにゆえ, the most polite of the four.