第四十天

[nnozomi]

是...的 for asking WH questions in the past tense (I think)
https://resources.allsetlearning.com/chinese/grammar/The_%22shi..._de%22_construction_for_emphasizing_details

Guardian:
你觉得她是怎么死的？ (a little graphic for first thing in the morning, but it's a perfect example...)
你是怎么知道的？(how did you know, how did you find out?)
说说吧，那些伤都是谁弄的？(also a little colorful...)

My practice:
这个汤是谁做的？很好吃。
你昨天是在哪里去的？
我们当时没知道，是怎么做好的？

Comments

[nnozomi]

I think so too! In an embedded question, no less. (Rebel quotes much appreciated! I should try to find more myself.)