Curious structure with 的 in conversation Above is an exchange taken from qiushibaike.com. As far as I can understand, it goes like this (trying to keep translation literal for example's sake):


  Son: Mom, our teacher's a liar!
  
  Mom: Why would you say that your teacher's a liar, did he cheat you or what?
  
  Son: I asked the teacher a question and he actually said that he doesn't 
  know! How can he know if I do it correctly or not if he doesn't know it?
  
  Mom: Well, son, when did you ask him that question?
  
  Son: ...During the test.


Now, the problematic sentence is this one:


  妈妈： 那儿子你是在什么时候问老师问题的？


My gripes:


  
  I would have said it: 『那儿子你什么时候问老师问题？』. Would that be correct? (do I need 在 before 什么时候 in there?)
  The sentence has 是 in it, which normally takes a noun phrase afterwards. Furthermore, to my knowledge, the possessive marker 的 is tacked at the end of a noun phrase. Is there a noun phrase in there, then? Is it 在什么时候问老师问题的？ If so, could you help me break down the sentence to understand its structure?
  If the sentence in (1) is correct, is the 在 in 在什么时候 always optional, or is it necessitated by the structure of the original sentence?
The link you attached has well explained 是....的 structure. 

Basically, 是......的 structure connotes past tense when used in the interrogative sentence. 

Say 你什么时候去？ vs 你是什么时候去的: the former means when will you go? The latter means when did you go? or when have you been (there)? 

A side note: sometimes 是 can be omitted in colloquial contexts. For example,  你什么时候去的， which is the same as 你是什么时候去的. 

Hope this could make it more clear to you.