since when have you been so nice这是个什么句子,具体分析下语法,谢谢

1。这是一个特殊疑问句,而疑问句序当然也就要用到倒装。
翻译：是从什么时候开始,你（变得）这么好/友善了?
2。因为疑问词when刚好做介词since的宾语,所以为了表明两者的紧密关系,就把since when一起提前了。
单独将when提前也可以,但是让看者很难分清since和when的关系。
如：I have been so nice since 2 years ago。
对2 years ago划线提问,则为：Since when have you been so nice?
3。再给楼主举些类似的句子：
What did you do it for?= For what did you do it?= Why did you do it?
With whom were you talking?= Who/Whom were you talking with?