▲你的错句错在将介词 since 放在句末。根据英语习惯，介词 since 通常不放在句末。又如：

他是从什么时候失踪的?

正：Since when has he been missing?  

误：When has he been missing since? 

你从什么时候开始感兴趣的？

正：Since when have you been interested?  

误：When have you been interested since? 

▲类似地，介词 but, during, except 通常也不位于句末。如：

那件事是在什么时期发生的?

正：During which period did it happen? 

误：Which period did it happen during? 

▲比较下面的句子（介词用在句末）：

What price did you sell it at? 它你卖什么价？

What do you use this thing for? 你用这东西来做什么？

A: Who’s that letter from? 那封信是谁寄来的?

B: Never you mind. 你少管闲事。