Why is " 这个意外什么时候发生？" not a valid answer?
I would think “When was it that this accident happened?” would be a better English translation given the 是。。。的 structure used.
I totally agree. The 是。。。的 format is used to emphasize whatever follows the 是. Translating from Chinese to English does not pose a problem, but given the current sentence in English, the 是。。。的 could be omitted.
This is an odd sentence 在 doesn't seem to fit. I would think 这个意外发生了什么时候 is an acceptable answer
I believe the time word (什么时候) needs to come before the verb (发生).
Otherwise, I agree with you.
Yes, what is the purpose of the 在?
Shouldn't 意外 be after 发生 according to the tips and notes?
这个意外什么时候发生了 should be marked as correct
什么时候发生了这个意外。。。should be accepted
这个意外在什么时候发生了。。。should be accepted... no need 是。。。的 structure