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

[elenothar]

I just went through that wiki entry and my brain is buzzing. Let's see if I retain all of that...

[nnozomi]

I know. The best I can do is "it's that..." for the statements (there's a Japanese construction which makes it easier to remember) and "use 是...的, not 了吗, for past WH questions," but I'm not at all sure that's either accurate or complete.